A lighthouse emitting light beams that are being obstructed by various sized blocks

Understanding Total Blocking Time in Lighthouse

When it comes to web performance, every second matters. The speed at which your website loads can significantly impact user experience, search engine rankings, and ultimately, your bottom line. One of the critical metrics to consider in this regard is the Total Blocking Time (TBT). This metric is part of Google’s Lighthouse suite of tools, which are designed to help developers improve the performance of their websites.

But what exactly is Total Blocking Time? How is it measured? And more importantly, how can you optimize it to improve your website’s performance? In this comprehensive guide, we will delve into these questions and more.

Defining Total Blocking Time

Total Blocking Time, or TBT, is a performance metric that quantifies the total amount of time that a page is blocked, preventing user interaction. This blockage occurs between First Contentful Paint (FCP) and Time to Interactive (TTI).

Essentially, TBT measures the total time that a page is “busy” and cannot respond to user inputs promptly. This could be due to long tasks, which are tasks that block the main thread for 50 milliseconds or more. The longer these tasks take, the higher the TBT.

Why is Total Blocking Time Important?

From a user’s perspective, a high TBT can lead to a frustrating experience. Imagine clicking on a button or trying to scroll down a page, and nothing happens. This delay in response can make users perceive your website as slow, even if the page loads visually quickly.

From a technical standpoint, TBT is a crucial metric because it directly impacts how search engines perceive your site. Google, for example, has incorporated TBT as one of the Core Web Vitals – a set of metrics that Google considers essential in a webpage’s overall user experience. Hence, a high TBT can negatively impact your site’s SEO ranking.

Measuring Total Blocking Time

Now that we understand what TBT is and why it’s important, let’s look at how it’s measured. As mentioned earlier, TBT is part of Google’s Lighthouse, an open-source, automated tool for improving the quality of web pages. You can run it against any webpage, public or requiring authentication.

Lighthouse calculates TBT by adding up the blocking portion of all long tasks between FCP and TTI. Any task that runs for more than 50 ms is considered a long task. The blocking portion is the amount of time in excess of 50 ms that a task runs.

How to Access Lighthouse and Run a Report

You can access Lighthouse in a few different ways. The easiest is probably through the Chrome DevTools. Simply right-click on any page you want to analyze, select ‘Inspect’, then go to the ‘Lighthouse’ tab and click ‘Generate report’.

Alternatively, you can also run Lighthouse from the command line, use it as a Node module, or even as a REST API. Regardless of the method, once the report is generated, you will see a section for ‘Performance’ where the TBT score is displayed.

Improving Total Blocking Time

Improving your TBT score essentially involves minimizing long tasks. This can be achieved through various strategies, including code splitting, optimizing your CSS, and offloading tasks to a web worker.

Code splitting involves breaking up your JavaScript into smaller, more manageable chunks that can be processed during idle times. This reduces the chance of long tasks blocking the main thread.

Optimizing CSS

CSS can often be a culprit for long tasks. Unoptimized CSS can block rendering and delay the user’s ability to interact with your page. To optimize your CSS, consider strategies like removing unused CSS, minifying CSS files, and using media queries to deliver appropriate stylesheets.

Offloading Tasks to a Web Worker

Web workers allow you to run JavaScript in the background, separate from the main execution thread. This means you can carry out complex computations or fetch data without blocking user interactions, thereby reducing TBT.


Understanding and optimizing Total Blocking Time is crucial in today’s web performance landscape. Not only does it play a significant role in user experience, but it also impacts your SEO ranking. By leveraging tools like Lighthouse and implementing strategies to minimize long tasks, you can improve your TBT score and, ultimately, the performance of your website.

Remember, a fast, responsive website leads to happy users, and happy users are more likely to become repeat visitors, subscribers, or customers. So, take the time to analyze your TBT and make the necessary improvements. Your users – and your bottom line – will thank you.

Leave a Comment

Your email address will not be published. Required fields are marked *

Click to access the login or register cheese