What is Total Blocking Time (TBT)?

Total Blocking Time (TBT) measures how long the page was frozen and unable to respond to taps or clicks while it loaded, because the browser was busy running JavaScript. It is the lab-test stand-in for real-world responsiveness:

• Good is under 200 milliseconds in a Lighthouse test.
• It predicts your INP - TBT is measured in the lab, INP on real visitors, but both come from the same cause: too much JavaScript hogging the main thread.
• The culprits are usually third-party - chat widgets, analytics, ad and tracking scripts all add blocking time.

TBT is the metric to watch when a page loads visually fast but feels unresponsive when you try to use it. Cut or defer the heavy scripts and TBT drops - and your real-world INP improves with it.