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Why does my site have a large total-blocking-time (tbt)?

Asked on Sep 17, 2025

Answer

Total Blocking Time (TBT) measures the time during which the main thread is blocked, preventing user interactions. A large TBT often indicates that your site has long-running tasks or heavy JavaScript execution that delays interactivity.

Example Concept: TBT is affected by tasks that take longer than 50ms to execute on the main thread. Optimizing TBT involves breaking up long tasks, deferring non-critical JavaScript, and minimizing the amount of JavaScript loaded initially to ensure the main thread remains responsive.

Additional Comment:

Analyze long tasks using the Performance panel in Chrome DevTools to identify scripts causing delays.
Consider code-splitting and lazy-loading JavaScript to reduce initial load time.
Use web workers to offload heavy computations from the main thread.
Ensure third-party scripts are loaded asynchronously to prevent blocking.

✅ Answered with Core Web Vitals best practices.

Ask any question about Performance here... and get an instant response.

Why does my site have a large total-blocking-time (tbt)?

Asked on Sep 17, 2025

Answer

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