Total Blocking Time (TBT) adds up the “blocking” part of every long task on the main thread between First Contentful Paint and Time to Interactive (TTI). Lighthouse stopped showing TTI as a metric in version 10, but still uses it as the end point for TBT. A long task is any task over 50 ms; only the time beyond 50 ms counts as blocking. While the main thread is blocked, the page cannot respond to input.
TBT carries the largest weight in the Lighthouse performance score (30% since Lighthouse 10, unchanged in Lighthouse 13), so it often explains a low Lighthouse score. On mobile, Lighthouse treats roughly 200 ms or less as good.
TBT is a lab metric. Its field counterpart is INP.
How to improve it
Ship less JavaScript, split large bundles, defer or remove third-party scripts, and move heavy work off the main thread or break it into smaller tasks.