Total Blocking Time - The metric to know for faster website performance

Feb 14, 2023 · 6 min read
Jacques Erasmus GitLab profile

Our world overwhelms us with information that is more accessible than ever. The increasing rates of content production and consumption are gifts that keep on giving. We can't seem to keep up with the information thrown at us. We're limited by our cognitive limitations and time constraints, and a recent study concluded the result is a shortening of attention spans. Websites are no exception.

Users who interact with your website want feedback, and want it fast. Preferably immediately! Website performance has become an important factor in keeping users engaged. But how do you measure how unresponsive a page is before it becomes fully interactive?

Many performance metrics exist, but this blog post focuses on Total Blocking Time (TBT).

What is Total Blocking Time?

TBT measures the total amount of time tasks were blocking your browser's main thread. This metric represents the total amount of time that a user could not interact with your website. It's measured between First Contentful Paint (FCP) and Time to Interactive (TTI), and represents the combined blocking time for all long tasks.

What is a long task?

A long task is a process that runs on the main thread for longer than 50 milliseconds (ms). After a task starts, a browser can't interrupt it, and a single long-running task can block the main thread. The result: a website that is unresponsive to user input until the task completes.

After the first 50 ms, all time spent on a task is counted as blocking time. This diagram shows five tasks, two of which block the main thread for 140 ms:

A diagram containing five tasks, two of which are blocking the main thread. The TBT for these tasks adds up to 140 ms.

How can we measure TBT?

Many tools measure TBT, but here we’ll use Chrome DevTools to analyze runtime performance.

As an example: We recently improved performance on GitLab's View Source page. This screenshot, taken before the performance improvement, shows eight long-running tasks containing a TBT of 2388.16 ms. That's more than two seconds:

A screenshot indicating that there are eight long-running tasks. The TBT of these tasks adds up to 2388.16 ms.

How can we improve TBT?

As you might have guessed by now, reducing the time needed to complete long-running tasks reduces TBT.

By selecting one of the tasks from the previous screenshot, we can get a breakdown of how the browser executed it. This Bottom-Up view shows that much time is spent on rendering content in the Document Object Model (DOM):

A screenshot of the Bottom-Up view of one of tasks from the previous screenshot. It indicates that most of the time is being spent on rendering content in the DOM.

This page has a lot of content that is below the fold – not immediately visible. The browser is spending a lot of resources upfront to render content that is not even visible to the user yet!

So what can we do? Some ideas:

Frameworks such as VueJS and React are already heavily optimized. However, be mindful of how you use these frameworks to avoid expensive tasks.

Change VueJS usage to improve TBT

This screenshot shows the Bottom-Up view of a task. Much of the task time is spent on activities from third-party code in the VueJS framework:

A screenshot of the Bottom-Up view of one of tasks. It indicates that a lot of the time is being spent on activities in the third-party VueJS framework.

What improvements can we make?

Results and methods

By using three of the methods suggested above, we reduced TBT from about 3 seconds to approximately 500 ms:

A chart indicating a drop in TBT from ~3 seconds to ~500 milliseconds.

What did we do?

Remember, the size of the decrease always depends on how optimized your app already is to begin with.

There is a lot more we can do to improve TBT. While the specific approach depends on the app you're optimizing, the general methods discussed here are very effective at finding improvement opportunities in any app. Like most things in life, a series of the smallest changes often yield the biggest impact. So let's iterate together, and adapt to this ever-changing world.

“Adaptability is the simple secret of survival.” – Jessica Hagedorn

Cover image by Growtika on Unsplash

“Learn how to identify and fix some root causes for high Total Blocking Time (TBT).” – Jacques Erasmus

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