Total Blocking Time

Total Blocking Time (TBT) is a time based metric that describes JavaScript main thread activity. It is useful for understanding how long a page is unable to respond to user input.

TBT is designed to be more reliable than Time to Interactive, which can show more variation due to rogue JavaScript tasks.

How is Total Blocking Time calculated?

Total Blocking Time totals the duration ”blocking period” of long JavaScript tasks that occur after First Contentful Paint and before Time to Interactive. The blocking period is the duration of a task that is over 50ms. For example, a task that is 120ms in duration will be calculated as 70 ms of blocking time.

Total Blocking time
The total blocking time for 2 tasks of 120 ms and 75 ms will be 95 ms.

In Calibre, Total blocking time is calculated by Lighthouse, which is part of our testing suite.

What Total Blocking Time measurement is fast?

Pages with < 300 ms of blocking time can be considered as fast, 300—600 ms as average where as durations over 600 ms are considered slow.

Total Blocking Time desired values showing anything under 300ms as good, between 300ms and 600ms as needs improvement, and above 600ms as poor.
Good TBT MeasurementPoor TBT Measurement
≤ 300ms> 600ms

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