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TBT - Total Blocking Time (Web Vitals)

TBT is documented on Google's Web Dev. This page summarizes that documentation.

Total Blocking Time (TBT) is an aggregated metric measuring how non-interactive a page is during loading. It quantifies how long the main thread remains blocked before the page becomes reliably interactive.

What is TBT?​

Total Blocking Time measures the total amount of time the main thread is blocked long enough that a user interaction would not produce a timely response.

These periods are measured between First Contentful Paint (FCP) and Time to Interactive (TTI) — that is, from when the first content appears to when the page becomes interactive.

The main thread is considered "blocked" when a task runs longer than 50ms. If the user interacts at that moment, the site won't respond until the task finishes, which is perceivable.

The blocking time for a task is the portion of its duration beyond 50ms. TBT is the sum of all blocking times.

Scoring​

Good< 300ms
Fairbetween 300ms and 600ms
Poor> 600ms