Definition
Total Blocking Time (TBT) measures the total duration during which the browser's main thread is occupied by long tasks (over 50ms each) between First Contentful Paint and Time to Interactive. During these blocking periods, the browser cannot respond to user interactions. TBT is a lab metric that correlates strongly with FID and INP measured in the field. Google Lighthouse recommends a TBT below 200ms. High TBT is usually caused by excessive JavaScript, heavy third-party scripts, or blocking CSS/JS parsing.
Key Points
- Measures total time the browser is blocked and can't respond to interactions
- Recommended threshold: below 200ms per Google Lighthouse
- Correlates strongly with FID and INP measured in the field
Practical Examples
Interactive lag diagnosis
A site shows 1,800ms TBT caused by a chat widget loading 500KB of JavaScript. Replacing it with a lighter solution reduces TBT to 150ms.
JavaScript optimization
Code-splitting and deferred loading of non-critical scripts reduces a SPA's TBT from 2,400ms to 180ms.
Frequently Asked Questions
TBT is a lab metric measuring total main thread blocking. FID is a field metric measuring the actual delay of a user's first interaction. TBT predicts FID well but they're not identical.
Reduce JavaScript size, use code-splitting, defer non-critical scripts with async/defer, and minimize third-party scripts. Avoid any JavaScript task over 50ms on the main thread.
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Last updated: 2026-02-07