Glossary: Every Web Performance Term, Explained
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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