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 above 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:
The timeline depicted in the preceding image has five tasks, three of which are Long Tasks because their duration exceeds 50 milliseconds. The next diagram shows the blocking time for each of the long tasks:
So while the total time spent running tasks on the main thread is 560 milliseconds, only 345 milliseconds of that time is considered blocking time.
| Task duration (milliseconds) | Task blocking time (milliseconds) | |
|---|---|---|
| Task one | 250 | 200 |
| Task two | 90 | 40 |
| Task three | 35 | 0 |
| Task four | 30 | 0 |
| Task five | 155 | 105 |
| Total Blocking Time | 345 milliseconds | |
How does TBT relate to TTI?
TBT is measured over a time period. For some lab tools that traditionally measure page loads, including Lighthouse, TBT has been measured up until TTI because it helps quantify the severity of how non-interactive a page is prior it to becoming reliably interactive. However, TBT could also continue to be measured after page load and so beyond TTI, for example in Lighthouse Timespan mode.
TTI considers a page "reliably interactive" if the main thread has been free of long tasks for at least five seconds. This means that three, 51 ms tasks spread out over 10 seconds can push back TTI just as far as a single 10-second long task—but those two scenarios would feel very different to a user trying to interact with the page.
In the first case, three, 51 ms tasks would have a TBT of 3 milliseconds. Whereas a single, 10-second long tasks would have a TBT of 9950 milliseconds. The larger TBT value in the second case quantifies the worse experience.
This example shows why TBT is often a better metric than TTI as it is less prone to outliers. This is even the case when TTI is used as the endpoint for TBT.
How to measure TBT
TBT is a metric that should be measured in the lab. The best way to measure TBT is to run a Lighthouse performance audit on your site. See the Lighthouse documentation on TBT for usage details.
Lab tools
What is a good TBT score?
To provide a good user experience, sites should strive to have a Total Blocking Time of less than 200 milliseconds when tested on average mobile hardware.
For details on how your page's TBT affects your Lighthouse performance score, see How Lighthouse determines your TBT score
How to improve TBT
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: