Total blocking time

Total Blocking Time (TBT): How it affects your website?

It’s great to create a website and move your business online. Almost everyone does the same but there are certain things that you don’t notice while you start with your awesome world of internet. One of them is Total Blocking Time (TBT)

The total blocking time is the matrix that is used to consider the performance of the website.

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.

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

What Total Blocking Time measurement is fast?

Pages with < 300 ms of blocking time can be considered as fast, 300—600 ms as average whereas durations over 600 ms are considered slow. Learn more about Total Blocking Time.

Good TBT MeasurementPoor TBT Measurement
≤ 300ms> 600ms

How to reduce Total Blocking Time ?

To learn how to improve TBT for a specific site, you can run a Lighthouse performance audit and pay attention to any specific opportunities the audit suggests.

To learn how to improve TBT in general (for any site), refer to the following performance guides:

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Vikash

Vikash Mishra has over 5+ years of experience in Web Development, Server management, and Networking industries. He has also been an educator on topics such as Web Development, Python, Server technologies. He believes in having fun in your work and enjoying what you do. He is passionate about IT systems, and believes strongly in passing this passion and skillset on to the next generation of IT enthusiast. View all posts by Vikash