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Squarespace Speed Optimization ](/services/squarespace-speed-optimization) [Book a Call](/contact) [Home](/) [Resources](/resources) [Glossary](/resources/glossary) Total Blocking Time Loading Performance · Glossary # Total Blocking Time (TBT) ![Matt Suffoletto](/assets/matt-CEkmAuBN.jpg)Written by [Matt Suffoletto](/resources/author/matt-suffoletto) Published June 26, 2026 ![Total Blocking Time](/__l5e/assets-v1/921dc4d3-eb48-4871-a6e7-06fc79d0e890/gloss-hero-total-blocking-time.png) Share 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Get my audit 100% free Results in 24h ## TL;DR 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. ## What is Total Blocking Time (TBT)? 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. Learn more in our [how to diagnose TBT](/resources/guides/ultimate-measurement-tools-data-guide) guide, or work with us through our [done-for-you speed optimization service](/services/core-web-vitals-optimization). ## TBT Thresholds & Benchmarks Metric Good Needs Improvement Poor TBT (lab) ≤ 200 ms 200 ms, 600 ms \> 600 ms ## How TBT is Measured TBT is calculated by Lighthouse and PageSpeed Insights from a recorded performance trace. ## Related Terms [Glossary ### Long Tasks ](/resources/glossary/long-tasks)[Glossary ### Main Thread Work ](/resources/glossary/main-thread-work)[Glossary ### Interaction to Next Paint (INP) ](/resources/glossary/interaction-to-next-paint) Sources - [web.dev: TBT](https://web.dev/articles/tbt) [![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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