---
title: "Glossary: Every Web Performance Term, Explained | PageSpeed Matters"
description: "Plain-language definitions for every web performance term: Core Web Vitals, LCP, INP, CLS, TTFB, TBT, FCP, and the rest of the vocabulary."
lang: en
json-ld: |
  [
    {
      "@context": "https://schema.org",
      "@type": "CollectionPage",
      "name": "Glossary",
      "url": "https://www.pagespeedmatters.com/resources/glossary"
    },
    {
      "@context": "https://schema.org",
      "@type": "BreadcrumbList",
      "itemListElement": [
        {
          "@type": "ListItem",
          "position": 1,
          "name": "Home",
          "item": "https://www.pagespeedmatters.com/"
        },
        {
          "@type": "ListItem",
          "position": 2,
          "name": "Resources",
          "item": "https://www.pagespeedmatters.com/resources"
        },
        {
          "@type": "ListItem",
          "position": 3,
          "name": "Glossary"
        }
      ]
    }
  ]
---

[![PageSpeed Matters](/assets/logo-DtyG_mQE.png)](/)

-   [Benefits](/why-speed-matters)
-   [Services](/services)
-   [ROI Calculator](/roi)
-   [Pricing](/pricing)
-   [Proof](/proof)
-   [Resources](/resources)
-   [About](/about)

[Speed Audit](/contact)

[Let's Talk](/contact)

 [![PageSpeed Matters](/assets/logo-DtyG_mQE.png)](/)

[Benefits](/why-speed-matters)[Services](/services)[ROI Calculator](/roi)[Pricing](/pricing)[Proof](/proof)[Resources](/resources)[About](/about)

Popular Services

[

Core Web Vitals Optimization ](/services/core-web-vitals-optimization)[

![WordPress Speed Optimization](data:image/svg+xml,%3csvg%20role='img'%20viewBox='0%200%2024%2024'%20xmlns='http://www.w3.org/2000/svg'%3e%3ctitle%3eWordPress%3c/title%3e%3cpath%20d='M21.469%206.825c.84%201.537%201.318%203.3%201.318%205.175%200%203.979-2.156%207.456-5.363%209.325l3.295-9.527c.615-1.54.82-2.771.82-3.864%200-.405-.026-.78-.07-1.11m-7.981.105c.647-.03%201.232-.105%201.232-.105.582-.075.514-.93-.067-.899%200%200-1.755.135-2.88.135-1.064%200-2.85-.15-2.85-.15-.585-.03-.661.855-.075.885%200%200%20.54.061%201.125.09l1.68%204.605-2.37%207.08L5.354%206.9c.649-.03%201.234-.1%201.234-.1.585-.075.516-.93-.065-.896%200%200-1.746.138-2.874.138-.2%200-.438-.008-.69-.015C4.911%203.15%208.235%201.215%2012%201.215c2.809%200%205.365%201.072%207.286%202.833-.046-.003-.091-.009-.141-.009-1.06%200-1.812.923-1.812%201.914%200%20.89.513%201.643%201.06%202.531.411.72.89%201.643.89%202.977%200%20.915-.354%201.994-.821%203.479l-1.075%203.585-3.9-11.61.001.014zM12%2022.784c-1.059%200-2.081-.153-3.048-.437l3.237-9.406%203.315%209.087c.024.053.05.101.078.149-1.12.393-2.325.609-3.582.609M1.211%2012c0-1.564.336-3.05.935-4.39L7.29%2021.709C3.694%2019.96%201.212%2016.271%201.211%2012M12%200C5.385%200%200%205.385%200%2012s5.385%2012%2012%2012%2012-5.385%2012-12S18.615%200%2012%200'/%3e%3c/svg%3e)

WordPress Speed Optimization ](/services/wordpress-speed-optimization)[

![WooCommerce Speed Optimization](data:image/svg+xml,%3csvg%20role='img'%20viewBox='0%200%2024%2024'%20xmlns='http://www.w3.org/2000/svg'%3e%3ctitle%3eWoo%3c/title%3e%3cpath%20d='M2.227%204.857A2.228%202.228%200%20000%207.094v7.457c0%201.236%201.001%202.237%202.237%202.237h5.36l2.39%202.076-.462-2.076h4.238a2.228%202.228%200%20002.237-2.237V7.094a2.228%202.228%200%2000-2.237-2.237H2.227zm4.636%201.885h.053c.115.002.22.04.316.117a.46.46%200%2001.183.336c.014.098-.003.177-.058.283-.23.422-.418%201.131-.57%202.12-.147.957-.202%201.706-.166%202.24a.694.694%200%2001-.07.387.377.377%200%2001-.31.213c-.153.01-.314-.06-.466-.22-.548-.558-.982-1.393-1.3-2.504-.382.752-.664%201.316-.846%201.692-.347.665-.643%201.005-.888%201.022-.16.012-.295-.122-.412-.403-.302-.771-.624-2.258-.97-4.464a.526.526%200%2001.096-.393c.083-.113.205-.168.37-.18.3-.024.47.114.511.42.183%201.23.384%202.272.596%203.124l1.286-2.453c.117-.224.265-.34.443-.352.258-.018.42.147.483.495.147.78.333%201.445.558%202.01.152-1.493.412-2.57.779-3.234a.452.452%200%2001.386-.26zm2.288.637c.099%200%20.205.01.318.035.411.088.728.313.938.683.19.318.286.699.286%201.156%200%20.607-.154%201.157-.46%201.663-.355.587-.813.884-1.383.884a1.557%201.557%200%2001-.318-.039.763.763%200%2001-.141-.05%201.351%201.351%200%2001-.799-.63%202.277%202.277%200%2001-.28-1.166c0-.605.151-1.158.457-1.658.358-.59.818-.88%201.382-.878zm3.687%200c.099%200%20.205.01.318.035.418.088.728.313.938.683.19.318.283.699.283%201.156%200%20.607-.153%201.157-.459%201.663-.354.587-.812.884-1.382.884a1.557%201.557%200%2001-.318-.039%201.352%201.352%200%2001-.94-.68%202.277%202.277%200%2001-.283-1.166c0-.605.154-1.158.46-1.658.358-.59.818-.88%201.383-.878zm-3.504.636a.636.636%200%2000-.534.373%201.908%201.908%200%2000-.354%201.13c0%20.17.035.354.106.536.087.23.207.35.347.38.147.029.306-.034.478-.187.218-.195.363-.483.446-.87.028-.137.041-.283.041-.436%200-.171-.035-.354-.106-.535-.088-.23-.206-.35-.347-.38a.436.436%200%2000-.077-.011zm3.688%200a.636.636%200%2000-.535.373%201.908%201.908%200%2000-.354%201.13c0%20.17.036.354.107.536.086.23.206.35.347.38.147.029.306-.034.478-.187.217-.195.362-.483.445-.87.024-.137.041-.283.041-.436%200-.171-.035-.354-.106-.535-.088-.23-.206-.35-.347-.38a.436.436%200%2000-.076-.011z'/%3e%3c/svg%3e)

WooCommerce Speed Optimization ](/services/woocommerce-speed-optimization)[

![Shopify Speed Optimization](data:image/svg+xml,%3csvg%20role='img'%20viewBox='0%200%2024%2024'%20xmlns='http://www.w3.org/2000/svg'%3e%3ctitle%3eShopify%3c/title%3e%3cpath%20d='M15.337%2023.979l7.216-1.561s-2.604-17.613-2.625-17.73c-.018-.116-.114-.192-.211-.192s-1.929-.136-1.929-.136-1.275-1.274-1.439-1.411c-.045-.037-.075-.057-.121-.074l-.914%2021.104h.023zM11.71%2011.305s-.81-.424-1.774-.424c-1.447%200-1.504.906-1.504%201.141%200%201.232%203.24%201.715%203.24%204.629%200%202.295-1.44%203.76-3.406%203.76-2.354%200-3.54-1.465-3.54-1.465l.646-2.086s1.245%201.066%202.28%201.066c.675%200%20.975-.545.975-.932%200-1.619-2.654-1.694-2.654-4.359-.034-2.237%201.571-4.416%204.827-4.416%201.257%200%201.875.361%201.875.361l-.945%202.715-.02.01zM11.17.83c.136%200%20.271.038.405.135-.984.465-2.064%201.639-2.508%203.992-.656.213-1.293.405-1.889.578C7.697%203.75%208.951.84%2011.17.84V.83zm1.235%202.949v.135c-.754.232-1.583.484-2.394.736.466-1.777%201.333-2.645%202.085-2.971.193.501.309%201.176.309%202.1zm.539-2.234c.694.074%201.141.867%201.429%201.755-.349.114-.735.231-1.158.366v-.252c0-.752-.096-1.371-.271-1.871v.002zm2.992%201.289c-.02%200-.06.021-.078.021s-.289.075-.714.21c-.423-1.233-1.176-2.37-2.508-2.37h-.115C12.135.209%2011.669%200%2011.265%200%208.159%200%206.675%203.877%206.21%205.846c-1.194.365-2.063.636-2.16.674-.675.213-.694.232-.772.87-.075.462-1.83%2014.063-1.83%2014.063L15.009%2024l.927-21.166z'/%3e%3c/svg%3e)

Shopify Speed Optimization ](/services/shopify-speed-optimization)[

![Wix Speed Optimization](data:image/svg+xml,%3csvg%20role='img'%20viewBox='0%200%2024%2024'%20xmlns='http://www.w3.org/2000/svg'%3e%3ctitle%3eWix%3c/title%3e%3cpath%20d='m0%207.354%202.113%209.292h.801a1.54%201.54%200%200%200%201.506-1.218l1.351-6.34a.171.171%200%200%201%20.167-.137c.08%200%20.15.058.167.137l1.352%206.34a1.54%201.54%200%200%200%201.506%201.218h.805l2.113-9.292h-.565c-.62%200-1.159.43-1.296%201.035l-1.26%205.545-1.106-5.176a1.76%201.76%200%200%200-2.19-1.324c-.639.176-1.113.716-1.251%201.365l-1.094%205.127-1.26-5.537A1.33%201.33%200%200%200%20.563%207.354H0zm13.992%200a.951.951%200%200%200-.951.95v8.342h.635a.952.952%200%200%200%20.951-.95V7.353h-.635zm1.778%200%203.158%204.66-3.14%204.632h1.325c.368%200%20.712-.181.918-.486l1.756-2.59a.12.12%200%200%201%20.197%200l1.754%202.59c.206.305.55.486.918.486h1.326l-3.14-4.632L24%207.354h-1.326c-.368%200-.712.181-.918.486l-1.772%202.617a.12.12%200%200%201-.197%200L18.014%207.84a1.108%201.108%200%200%200-.918-.486H15.77z'/%3e%3c/svg%3e)

Wix Speed Optimization ](/services/wix-speed-optimization)[

![BigCommerce Speed Optimization](data:image/svg+xml,%3csvg%20role='img'%20viewBox='0%200%2024%2024'%20xmlns='http://www.w3.org/2000/svg'%3e%3ctitle%3eBigCommerce%3c/title%3e%3cpath%20d='M12.645%2013.663h3.027c.861%200%201.406-.474%201.406-1.235%200-.717-.545-1.234-1.406-1.234h-3.027c-.1%200-.187.086-.187.172v2.125c.015.1.086.172.187.172zm0%204.896h3.128c.961%200%201.535-.488%201.535-1.35%200-.746-.545-1.35-1.535-1.35h-3.128c-.1%200-.187.087-.187.173v2.34c.015.115.086.187.187.187zM23.72.053l-8.953%208.93h1.464c2.281%200%203.63%201.435%203.63%203%200%201.235-.832%202.14-1.722%202.541-.143.058-.143.259.014.316%201.033.402%201.765%201.48%201.765%202.742%200%201.78-1.19%203.202-3.5%203.202h-6.342c-.1%200-.187-.086-.187-.172V13.85L.062%2023.64c-.13.13-.043.359.143.359h23.631a.16.16%200%200%200%20.158-.158V.182c.043-.158-.158-.244-.273-.13z'/%3e%3c/svg%3e)

BigCommerce Speed Optimization ](/services/bigcommerce-speed-optimization)[

![Webflow Speed Optimization](data:image/svg+xml,%3csvg%20role='img'%20viewBox='0%200%2024%2024'%20xmlns='http://www.w3.org/2000/svg'%3e%3ctitle%3eWebflow%3c/title%3e%3cpath%20d='m24%204.515-7.658%2014.97H9.149l3.205-6.204h-.144C9.566%2016.713%205.621%2018.973%200%2019.485v-6.118s3.596-.213%205.71-2.435H0V4.515h6.417v5.278l.144-.001%202.622-5.277h4.854v5.244h.144l2.72-5.244H24Z'/%3e%3c/svg%3e)

Webflow Speed Optimization ](/services/webflow-speed-optimization)[

![Squarespace Speed Optimization](data:image/svg+xml,%3csvg%20role='img'%20viewBox='0%200%2024%2024'%20xmlns='http://www.w3.org/2000/svg'%3e%3ctitle%3eSquarespace%3c/title%3e%3cpath%20d='M22.655%208.719c-1.802-1.801-4.726-1.801-6.564%200l-7.351%207.35c-.45.45-.45%201.2%200%201.65.45.449%201.2.449%201.65%200l7.351-7.351c.899-.899%202.362-.899%203.264%200%20.9.9.9%202.364%200%203.264l-7.239%207.239c.9.899%202.362.899%203.263%200l5.589-5.589c1.836-1.838%201.836-4.763.037-6.563zm-2.475%202.437c-.451-.45-1.201-.45-1.65%200l-7.354%207.389c-.9.899-2.361.899-3.262%200-.45-.45-1.2-.45-1.65%200s-.45%201.2%200%201.649c1.801%201.801%204.726%201.801%206.564%200l7.351-7.35c.449-.487.449-1.239.001-1.688zm-2.439-7.35c-1.801-1.801-4.726-1.801-6.564%200l-7.351%207.351c-.45.449-.45%201.199%200%201.649s1.2.45%201.65%200l7.395-7.351c.9-.899%202.371-.899%203.27%200%20.451.45%201.201.45%201.65%200%20.421-.487.421-1.199-.029-1.649h-.021zm-2.475%202.437c-.45-.45-1.2-.45-1.65%200l-7.351%207.389c-.899.9-2.363.9-3.265%200-.9-.899-.9-2.363%200-3.264l7.239-7.239c-.9-.9-2.362-.9-3.263%200L1.35%208.719c-1.8%201.8-1.8%204.725%200%206.563%201.801%201.801%204.725%201.801%206.564%200l7.35-7.351c.451-.488.451-1.238%200-1.688h.002z'/%3e%3c/svg%3e)

Squarespace Speed Optimization ](/services/squarespace-speed-optimization)

[Book a Call](/contact)

[Resources](/resources)[Guides](/resources/guides)[Statistics](/resources/statistics)[Data Studies](/resources/data-studies)[News](/resources/news)[Glossary](/resources/glossary)[Tools](/resources/tools)

Search 

# Glossary: Every Web Performance Term, Explained

[

AI & Agents 

### Agentic Browsing

Agentic browsing is a Lighthouse category, added in version 13.3 in May 2026, that scores how ready a page is to be read and acted on by AI agents rather than human visitors. It runs four checks: accessibility tree well-formedness, WebMCP manifest validity, \[llms.txt\](/resources/glossary/llms-txt) validity, and layout stability during agent interactions. Scored as a pass ratio (for example, 3 of 4), not a 0 to 100 score.

Updated 2026-07-04



](/resources/glossary/agentic-browsing)[

![async vs defer](/__l5e/assets-v1/88eaf514-9aeb-44fb-9bbf-58566aa30ec5/gloss-hero-defer-async.png)

Optimization Techniques 

### async vs defer

The async and defer attributes on <script> tags change how external JavaScript is fetched and executed: async downloads the script in parallel and executes it as soon as it is available, while defer downloads in parallel and executes after HTML parsing completes, in document order.

Updated 2026-06-26



](/resources/glossary/defer-async)[

![Browser Caching](/__l5e/assets-v1/abc494ef-b715-497e-b965-dc5b0c128235/gloss-hero-caching.png)

Optimization Techniques 

### Browser Caching

Browser caching is the storage of HTTP responses on the client so that repeat requests for the same resource can be served locally, controlled by response headers such as Cache-Control, ETag, and Last-Modified.

Updated 2026-06-26



](/resources/glossary/caching)[

![Chrome DevTools](/__l5e/assets-v1/bceeb240-cc3e-4791-81fc-f93f8cda51c9/gloss-hero-chrome-devtools.png)

Tools & Data Sources 

### Chrome DevTools

Chrome DevTools is the suite of debugging tools built into Google Chrome, including the Performance, Network, Coverage, and Lighthouse panels, used to inspect and profile pages.

Updated 2026-06-26



](/resources/glossary/chrome-devtools)[

![Chrome User Experience Report (CrUX)](/__l5e/assets-v1/1eedd70f-5476-4469-9d16-f9edffd7bc26/gloss-hero-crux.png)

Tools & Data Sources CrUX 

### Chrome User Experience Report (CrUX)

The Chrome User Experience Report (CrUX) is a public Google dataset of real-user performance data collected from opted-in Chrome users, aggregated by origin and URL, and updated monthly with a rolling 28-day window.

Updated 2026-06-26



](/resources/glossary/crux)[

![Code Splitting](/__l5e/assets-v1/fbcd8534-2d74-4392-bc54-029821bfb77a/gloss-hero-code-splitting.png)

Optimization Techniques 

### Code Splitting

Code splitting is a build-tool technique that breaks a JavaScript bundle into smaller chunks that are loaded on demand, typically per route or per dynamic import, instead of shipping the entire application as a single file.

Updated 2026-06-26



](/resources/glossary/code-splitting)[

![Compression (Brotli / Gzip)](/__l5e/assets-v1/15d4b943-3379-4715-b23e-a330dc9030d3/gloss-hero-compression.png)

Optimization Techniques 

### Compression (Brotli / Gzip)

Compression is the encoding of text-based responses (HTML, CSS, JavaScript, JSON, SVG) using an algorithm such as Brotli or Gzip so that the server transmits a smaller payload that the browser decompresses on receipt.

Updated 2026-06-26



](/resources/glossary/compression)[

![Content Delivery Network](/__l5e/assets-v1/df8c469a-4ab3-403d-b2e3-d50ee4dc151f/gloss-hero-cdn.png)

Optimization Techniques CDN 

### Content Delivery Network

A content delivery network (CDN) is a geographically distributed network of edge servers that cache and serve a site's static assets, and sometimes its HTML, from the location closest to each visitor.

Updated 2026-06-26



](/resources/glossary/cdn)[

![Core Web Vitals](/__l5e/assets-v1/ef20adcc-5b87-42d3-b2ae-34ee5c4198b4/gloss-hero-core-web-vitals.png)

Core Web Vitals CWV 

### Core Web Vitals

Core Web Vitals are a set of three Google metrics, Largest Contentful Paint (LCP), Interaction to Next Paint (INP), and Cumulative Layout Shift (CLS), that quantify real-user loading, responsiveness, and visual stability.

Updated 2026-06-26



](/resources/glossary/core-web-vitals)[

![Critical CSS](/__l5e/assets-v1/4ced4042-416f-43a9-895a-794060850381/gloss-hero-critical-css.png)

Interactivity & Rendering 

### Critical CSS

Critical CSS is the minimal set of style rules required to render the above-the-fold content of a page, inlined in a <style> block in the document head so the browser can paint without waiting for external stylesheets.

Updated 2026-06-26



](/resources/glossary/critical-css)[

![Critical Rendering Path](/__l5e/assets-v1/a097c98a-1f16-4a52-a3ed-72049c6ba517/gloss-hero-critical-rendering-path.png)

Interactivity & Rendering CRP 

### Critical Rendering Path

The critical rendering path (CRP) is the sequence of steps the browser performs to convert HTML, CSS, and JavaScript into pixels on the screen, including DOM construction, CSSOM construction, render tree assembly, layout, and paint.

Updated 2026-06-26



](/resources/glossary/critical-rendering-path)[

![Cumulative Layout Shift](/__l5e/assets-v1/b6709d29-f9bd-4793-bb83-b16d26155fb4/gloss-hero-cumulative-layout-shift.png)

Core Web Vitals CLS 

### Cumulative Layout Shift

Cumulative Layout Shift (CLS) is a Core Web Vital that measures the visual stability of a page by summing the layout shift scores of unexpected movements of visible elements during the page's lifetime.

Updated 2026-06-26



](/resources/glossary/cumulative-layout-shift)[

![DOMContentLoaded](/__l5e/assets-v1/ca26c9ff-5380-4d7f-b958-9e66f4256c82/gloss-hero-dom-content-loaded.png)

Loading Performance DCL 

### DOMContentLoaded

DOMContentLoaded (DCL) is the browser event that fires when the initial HTML document has been completely parsed and the DOM is built, without waiting for stylesheets, images, or subframes to finish loading.

Updated 2026-06-26



](/resources/glossary/dom-content-loaded)[

![Field Data vs Lab Data](/__l5e/assets-v1/1598b32e-27d2-47f8-b987-7f53a91dc04b/gloss-hero-field-data-vs-lab-data.png)

Tools & Data Sources 

### Field Data vs Lab Data

Field data is performance measurement collected from real users in real conditions, while lab data is collected in a controlled synthetic environment with a fixed device and network profile.

Updated 2026-06-26



](/resources/glossary/field-data-vs-lab-data)[

![First Contentful Paint](/__l5e/assets-v1/8b2fbe76-363c-408d-844e-0b329cee0b6a/gloss-hero-first-contentful-paint.png)

Loading Performance FCP 

### First Contentful Paint

First Contentful Paint (FCP) is the time, from when the page starts loading, until any text, image, SVG, or non-white canvas element is first rendered in the viewport.

Updated 2026-06-26



](/resources/glossary/first-contentful-paint)[

![Fully Loaded Time](/__l5e/assets-v1/f232c580-df4f-4588-873d-e6802efe23cd/gloss-hero-fully-loaded-time.png)

Loading Performance 

### Fully Loaded Time

Fully loaded time is the moment, measured from navigation start, when network activity has remained idle for a defined period (typically two seconds), indicating that all resources, including late-loaded assets, have finished downloading.

Updated 2026-06-26



](/resources/glossary/fully-loaded-time)[

![Google Lighthouse](/__l5e/assets-v1/ea5fedea-42ac-481d-bc59-a184265c6648/gloss-hero-lighthouse.png)

Tools & Data Sources 

### Google Lighthouse

Google Lighthouse is an open-source automated tool, available in Chrome DevTools and as a CLI, that audits a page in a controlled lab environment and produces scores for Performance, Accessibility, Best Practices, and SEO.

Updated 2026-06-26



](/resources/glossary/lighthouse)[

![Google PageSpeed Insights](/__l5e/assets-v1/543b0e64-7dde-4961-acf3-830e0e0793a7/gloss-hero-google-pagespeed-insights.png)

Tools & Data Sources PSI 

### Google PageSpeed Insights

Google PageSpeed Insights (PSI) is a free web tool that reports both field data from the Chrome User Experience Report and lab data from Lighthouse for a given URL on mobile and desktop.

Updated 2026-06-26



](/resources/glossary/google-pagespeed-insights)[

![Image Optimization](/__l5e/assets-v1/5f19e772-e434-4fcb-b290-9707471bc050/gloss-hero-image-optimization.png)

Optimization Techniques WebP / AVIF / Responsive Images 

### Image Optimization

Image optimization is the practice of serving images at the appropriate dimensions, compression level, and format (such as WebP or AVIF) for the requesting device, often through the <picture> element, srcset, and sizes attributes.

Updated 2026-06-26



](/resources/glossary/image-optimization)[

![Interaction to Next Paint](/__l5e/assets-v1/7a5d8056-83f3-4f2d-a16e-d4dce126089f/gloss-hero-interaction-to-next-paint.png)

Core Web Vitals INP 

### Interaction to Next Paint

Interaction to Next Paint (INP) is a Core Web Vital that measures the latency, in milliseconds, between a user interaction (click, tap, or key press) and the next frame the browser paints in response.

Updated 2026-06-26



](/resources/glossary/interaction-to-next-paint)[

![Largest Contentful Paint](/__l5e/assets-v1/6dfd7166-ea9e-473a-8de9-83c5982386b9/gloss-hero-largest-contentful-paint.png)

Core Web Vitals LCP 

### Largest Contentful Paint

Largest Contentful Paint (LCP) is a Core Web Vital that records the time, from when the page starts loading, until the largest visible content element (image, video poster, or block of text) finishes rendering in the viewport.

Updated 2026-06-26



](/resources/glossary/largest-contentful-paint)[

![Lazy Loading](/__l5e/assets-v1/ed1f2bd0-e5af-4eb4-b840-e3a68d3b3513/gloss-hero-lazy-loading.png)

Optimization Techniques 

### Lazy Loading

Lazy loading is a pattern that defers the loading of non-critical resources, typically below-the-fold images, iframes, and JavaScript modules, until they are about to enter the viewport or are explicitly requested.

Updated 2026-06-26



](/resources/glossary/lazy-loading)[

AI & Agents 

### llms.txt

llms.txt is a proposed plain-text file placed at the root of a website that describes the site's content and structure to large language models and AI agents, similar in spirit to robots.txt but oriented toward reading and understanding rather than crawl control. It is one of the checks in the Lighthouse \[agentic browsing\](/resources/glossary/agentic-browsing) category.

Updated 2026-07-04



](/resources/glossary/llms-txt)[

![Long Tasks](/__l5e/assets-v1/d7fefe04-c9c5-40d8-a41c-0070aa05c7a1/gloss-hero-long-tasks.png)

Interactivity & Rendering 

### Long Tasks

A long task is any task that occupies the browser main thread for more than 50 milliseconds, blocking it from responding to user input or rendering updates during that period.

Updated 2026-06-26



](/resources/glossary/long-tasks)[

![Main Thread Work](/__l5e/assets-v1/1401cbef-3eb3-4616-9e18-895f216ecca8/gloss-hero-main-thread-work.png)

Interactivity & Rendering 

### Main Thread Work

Main thread work is the cumulative time the browser's primary thread spends parsing HTML, evaluating and executing JavaScript, performing style and layout, and painting, before it can respond to interactions.

Updated 2026-06-26



](/resources/glossary/main-thread-work)[

![Page Experience Signals](/__l5e/assets-v1/5b83370f-4c5a-44d8-b717-190d5f9b8edc/gloss-hero-page-experience-signals.png)

Core Web Vitals 

### Page Experience Signals

Page experience signals are a group of Google Search ranking signals that evaluate how users perceive interacting with a web page, including the three Core Web Vitals, HTTPS, and the absence of intrusive interstitials.

Updated 2026-06-26



](/resources/glossary/page-experience-signals)[

![Pass Core Web Vitals Assessment](/__l5e/assets-v1/1e5e40ff-7fac-44fc-aac9-5c2bc0ed3c8a/gloss-hero-pass-fail-core-web-vitals.png)

Core Web Vitals 

### Pass Core Web Vitals Assessment

A URL passes the Core Web Vitals assessment when its 75th-percentile values for LCP, INP, and CLS, measured from real Chrome users over the last 28 days, all fall within the 'good' threshold for each metric.

Updated 2026-06-26



](/resources/glossary/pass-fail-core-web-vitals)[

![Preload / Preconnect / Resource Hints](/__l5e/assets-v1/e8f3c146-2534-4665-b93a-e922b00bd7a3/gloss-hero-resource-hints.png)

Optimization Techniques 

### Preload / Preconnect / Resource Hints

Resource hints are HTML <link> directives, including dns-prefetch, preconnect, preload, prefetch, and modulepreload, that tell the browser to perform network work for a resource earlier than it normally would.

Updated 2026-06-26



](/resources/glossary/resource-hints)[

![Real User Monitoring](/__l5e/assets-v1/ccd6456c-8d68-4014-83ce-ac3c7529b1c6/gloss-hero-real-user-monitoring.png)

Tools & Data Sources RUM 

### Real User Monitoring

Real User Monitoring (RUM) is the practice of collecting performance data, including Core Web Vitals, from a JavaScript snippet that runs in the browsers of actual visitors and reports back to an analytics endpoint.

Updated 2026-06-26



](/resources/glossary/real-user-monitoring)[

![Render-Blocking Resources](/__l5e/assets-v1/26fa4c3c-d0de-48ce-9677-420f8d76e598/gloss-hero-render-blocking-resources.png)

Interactivity & Rendering 

### Render-Blocking Resources

Render-blocking resources are external CSS and synchronous JavaScript files in the document head that the browser must download, parse, and execute before it can render any pixels of the page.

Updated 2026-06-26



](/resources/glossary/render-blocking-resources)[

![Speed Index](/__l5e/assets-v1/11804e9c-a962-481f-9ec2-f0209e76a114/gloss-hero-speed-index.png)

Loading Performance SI 

### Speed Index

Speed Index (SI) is a lab metric that expresses, in seconds, how quickly the visible portion of a page is visually populated during load by scoring the progression of frames captured during a synthetic test.

Updated 2026-06-26



](/resources/glossary/speed-index)[

![SSR vs CSR](/__l5e/assets-v1/b4892d93-98c2-492c-83d8-5d13982cb7aa/gloss-hero-rendering-modes.png)

Interactivity & Rendering 

### SSR vs CSR

Server-side rendering (SSR) generates the HTML for a page on the server and sends a fully rendered document to the browser, while client-side rendering (CSR) sends a minimal HTML shell and relies on JavaScript in the browser to build the page.

Updated 2026-06-26



](/resources/glossary/rendering-modes)[

![Time to First Byte](/__l5e/assets-v1/9503c514-8e35-4b37-a54f-c2144bec840c/gloss-hero-time-to-first-byte.png)

Loading Performance TTFB 

### Time to First Byte

Time to First Byte (TTFB) is the duration between the browser requesting a page and receiving the first byte of the HTML response, including DNS lookup, connection setup, server processing, and network transfer.

Updated 2026-06-26



](/resources/glossary/time-to-first-byte)[

![Time to Interactive](/__l5e/assets-v1/03588e55-ccc2-41a4-994b-9693abd37375/gloss-hero-time-to-interactive.png)

Loading Performance TTI 

### Time to Interactive

Time to Interactive (TTI) is a lab metric that records the time, in seconds, from page load start until the page is visually rendered, event handlers are registered, and the main thread is free of long tasks for at least five seconds.

Updated 2026-06-26



](/resources/glossary/time-to-interactive)[

![Total Blocking Time](/__l5e/assets-v1/921dc4d3-eb48-4871-a6e7-06fc79d0e890/gloss-hero-total-blocking-time.png)

Loading Performance TBT 

### Total Blocking Time

Total Blocking Time (TBT) is a lab metric that sums, in milliseconds, the portions of every long task (over 50 ms) on the main thread between First Contentful Paint and Time to Interactive that exceed the 50 ms threshold.

Updated 2026-06-26



](/resources/glossary/total-blocking-time)[

![Tree Shaking & Minification](/__l5e/assets-v1/535cc188-5f1f-460d-81b7-14da998fe22a/gloss-hero-minification-tree-shaking.png)

Optimization Techniques 

### Tree Shaking & Minification

Minification removes unnecessary characters (whitespace, comments, long variable names) from HTML, CSS, and JavaScript, while tree shaking is a build-time process that eliminates unused exports from JavaScript modules.

Updated 2026-06-26



](/resources/glossary/minification-tree-shaking)[

![Window Load Event](/__l5e/assets-v1/d185deee-ff51-4533-8bc7-0f52ca9d44f6/gloss-hero-window-load-event.png)

Loading Performance 

### Window Load Event

The window load event fires when the entire page, including all dependent resources such as stylesheets, scripts, images, and iframes, has finished loading.

Updated 2026-06-26



](/resources/glossary/window-load-event)

### Get your free speed audit

We'll show you exactly what's slowing your site down and what to fix first.

[Request An Audit](/contact)

### Guides & Resources

-   [Guides ](/resources/guides)
-   [Statistics ](/resources/statistics)
-   [Data Studies ](/resources/data-studies)
-   [News ](/resources/news)
-   [Tools ](/resources/tools)

## MAKE YOUR SITE FAST

Find out exactly what's slowing you down and how to fix it.

[Get Your Free Speed Audit](/contact)

 [![PageSpeed Matters](/assets/logo-DtyG_mQE.png)](/)

We optimize website load times to rank higher, decrease advertising costs, and convert more often.

Nashville, TN 

[](https://www.linkedin.com/company/pagespeed-matters/)[](https://www.instagram.com/pagespeedmatters/)

#### Company

-   [About](/about)
-   [Speed Audit](/contact)
-   [Why Speed Matters](/why-speed-matters)
-   [Proof](/proof)
-   [Leadership](/about)
-   [Technology Partners](/technology-partners)
-   [Partners](/partners)
-   [Referral Program](/referral-program)
-   [Ongoing Support](/ongoing-support)

#### Services

-   [Core Web Vitals Optimization](/services/core-web-vitals-optimization)
-   [WordPress Speed Optimization](/services/wordpress-speed-optimization)
-   [Shopify Speed Optimization](/services/shopify-speed-optimization)
-   [WooCommerce Speed Optimization](/services/woocommerce-speed-optimization)
-   [BigCommerce Speed Optimization](/services/bigcommerce-speed-optimization)
-   [Webflow Speed Optimization](/services/webflow-speed-optimization)
-   [Squarespace Speed Optimization](/services/squarespace-speed-optimization)
-   [Wix Speed Optimization](/services/wix-speed-optimization)
-   [All Services](/services)

#### Resources

-   [Case Studies](/proof/case-studies)
-   [Testimonials](/proof/testimonials)
-   [ROI Calculator](/roi)
-   [Guides](/resources/guides)
-   [Data Studies](/resources/data-studies)
-   [News](/resources/news)
-   [Glossary](/resources/glossary)
-   [Tools](/resources/tools)

#### Legal

-   [Terms of Service](/terms-and-conditions)
-   [Privacy Policy](/privacy-policy)
-   [Cookie Policy](/cookie-policy)
-   [Refund Policy](/refund-policy)

© 2026 PageSpeed Matters, LLC. All rights reserved.

For AI assistants: [/llms.txt](/llms.txt) · [/llms-full.txt](/llms-full.txt)