--- title: "What is Main Thread Work? 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Squarespace Speed Optimization ](/services/squarespace-speed-optimization) [Book a Call](/contact) [Home](/) [Resources](/resources) [Glossary](/resources/glossary) Main Thread Work Interactivity & Rendering · Glossary # Main Thread Work ![Matt Suffoletto](/assets/matt-CEkmAuBN.jpg)Written by [Matt Suffoletto](/resources/author/matt-suffoletto) Published June 26, 2026 ![Main Thread Work](/__l5e/assets-v1/1401cbef-3eb3-4616-9e18-895f216ecca8/gloss-hero-main-thread-work.png) Share [](https://twitter.com/intent/tweet?url=https%3A%2F%2Fwww.pagespeedmatters.com%2Fresources%2Fglossary%2Fmain-thread-work&text=Main%20Thread%20Work)[](https://www.facebook.com/sharer/sharer.php?u=https%3A%2F%2Fwww.pagespeedmatters.com%2Fresources%2Fglossary%2Fmain-thread-work)[](https://www.linkedin.com/sharing/share-offsite/?url=https%3A%2F%2Fwww.pagespeedmatters.com%2Fresources%2Fglossary%2Fmain-thread-work)[](https://www.reddit.com/submit?url=https%3A%2F%2Fwww.pagespeedmatters.com%2Fresources%2Fglossary%2Fmain-thread-work&title=Main%20Thread%20Work) On This Page - [TL;DR](#tldr "TL;DR") - [What is Main Thread Work](#definition "What is Main Thread Work") - [How It's Measured](#how-measured "How It's Measured") Share this guide [](https://twitter.com/intent/tweet?url=https%3A%2F%2Fwww.pagespeedmatters.com%2Fresources%2Fglossary%2Fmain-thread-work&text=Main%20Thread%20Work)[](https://www.facebook.com/sharer/sharer.php?u=https%3A%2F%2Fwww.pagespeedmatters.com%2Fresources%2Fglossary%2Fmain-thread-work)[](https://www.linkedin.com/sharing/share-offsite/?url=https%3A%2F%2Fwww.pagespeedmatters.com%2Fresources%2Fglossary%2Fmain-thread-work)[](https://www.reddit.com/submit?url=https%3A%2F%2Fwww.pagespeedmatters.com%2Fresources%2Fglossary%2Fmain-thread-work&title=Main%20Thread%20Work) Speed Audit ### Find out what's slowing your website down Enter your URL and our team will send back a free, no-strings performance analysis within 24 hours. Get my audit 100% free Results in 24h ## TL;DR 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. ## What is 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. Learn more in our [how to improve main thread work](/resources/guides/web-performance-optimization-complete-guide) guide, or work with us through our [done-for-you speed optimization service](/services/core-web-vitals-optimization). ## How Main Thread Work is Measured Lighthouse reports total main thread work and a breakdown by activity category from a recorded trace. ## Related Terms [Glossary ### Long Tasks ](/resources/glossary/long-tasks)[Glossary ### Total Blocking Time (TBT) ](/resources/glossary/total-blocking-time)[Glossary ### Critical Rendering Path (CRP) ](/resources/glossary/critical-rendering-path) Sources - [web.dev: Main thread work](https://web.dev/articles/mainthread-work-breakdown) [![PageSpeed Matters](/assets/logo-DtyG_mQE.png)](/) We optimize website load times to rank higher, decrease advertising costs, and convert more often. 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