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 guide, or work with us through our done-for-you speed optimization service.
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.