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Total Blocking Time
Also known as: TBT
The total time the main thread is too busy to respond to user input — measured by Lighthouse, not by real users.
Total Blocking Time sums the duration of every long task on the main thread between First Contentful Paint and Time to Interactive. A long task is any single block of JavaScript execution longer than 50ms — long enough that the browser can’t respond to a click or scroll until it finishes.
A fast TBT is under 200ms. Anything over 600ms means the site feels unresponsive even when it looks visually complete. The fix is almost always reducing the amount of JavaScript that has to parse and execute on first load: code-splitting, lazy-loading non-critical components, smaller bundles.
TBT is a lab metric — Lighthouse measures it on a synthetic load — and field equivalents like Interaction to Next Paint (INP) are what real users actually feel. They usually move together, but if you’re optimizing for one and ignoring the other, you’ll get a surprise in production.
Specs & references