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Author: Admin | 2025-04-28
What is TBT?The Total Blocking Time (TBT) metric measures the total amount of time after First Contentful Paint (FCP) where the main thread was blocked for long enough to prevent input responsiveness.By default, Lighthouse stops monitoring TBT after Time to Interactive (TTI), as do some other lab tools that measure page load. See How does TBT relate to TTI?.The main thread is considered "blocked" any time there's a Long Task a task that runs on the main thread for more than 50 milliseconds. We say the main thread is "blocked" because the browser cannot interrupt a task that's in progress. So in the event that a user does interact with the page in the middle of a long task, the browser must wait for the task to finish before it can respond.If the task is long enough (anything higher than 50 milliseconds), it's likely that the user will notice the delay and perceive the page as sluggish or broken.The blocking time of a given long task is its duration in excess of 50 milliseconds. And the total blocking time for a page is the sum of the blocking time for each long task that occurs after FCP for the measured timeframe (typically TTI for page load tools, or the total trace time for other tooling).For example, consider the following diagram of the browser's main thread during page load: A timeline of tasks on the main thread. The timeline depicted in the preceding image has five tasks, three of which are
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