A stopwatch surrounded by various website elements like loading bars

Understanding Total Blocking Time

Web performance metrics have become an integral part of the digital landscape. Among these metrics, Total Blocking Time (TBT) stands out as a crucial factor in measuring the responsiveness of a webpage. But what exactly is Total Blocking Time, and why does it matter? Let’s delve into the details.

What is Total Blocking Time?

Total Blocking Time, or TBT, is a performance metric that quantifies the total amount of time between First Contentful Paint (FCP) and Time to Interactive (TTI) where the main thread was blocked for long enough to prevent input responsiveness. In simpler terms, it measures the total time when a page is “busy” and cannot respond to user interactions promptly.

It’s important to note that TBT only considers tasks that block the main thread for more than 50 milliseconds. Shorter tasks don’t significantly impact the user’s experience, so they’re not included in the TBT calculation.

Why is Total Blocking Time Important?

Understanding TBT is crucial because it directly impacts the user experience. A high TBT means that users may experience delays when interacting with your webpage. This can lead to frustration and potentially cause users to leave your site.

Moreover, TBT is one of the Core Web Vitals that Google uses to assess the overall user experience provided by a webpage. Therefore, optimizing TBT can also contribute to improved SEO rankings.

How to Measure Total Blocking Time

Using Lighthouse

Lighthouse is a popular open-source tool for assessing webpage performance. It provides a detailed report that includes TBT among other metrics. To use Lighthouse, you can run it in Chrome DevTools, from the command line, or as a Node module.

After running Lighthouse, you’ll find the TBT value in the “Performance” section of the report. It’s expressed in milliseconds and should ideally be as low as possible.

Using WebPageTest

WebPageTest is another tool that can measure TBT. It provides a waterfall view of your webpage’s activity, making it easy to identify long tasks that contribute to a high TBT. To use WebPageTest, simply enter your webpage’s URL and select a test location and browser.

After running the test, you’ll find the TBT value under the “Timing Breakdown” section. Like with Lighthouse, a lower TBT value is better.

How to Optimize Total Blocking Time

Minimize Long Tasks

Since TBT measures the total time when the main thread is blocked by long tasks, reducing the number and duration of these tasks is a direct way to improve TBT. This can be achieved by breaking down long tasks into smaller, asynchronous ones that don’t block the main thread.

Common sources of long tasks include heavy JavaScript execution, large images or assets, and third-party scripts. Optimizing these elements can significantly reduce TBT.

Optimize JavaScript

JavaScript is often the main culprit behind long tasks. Therefore, optimizing your JavaScript can have a substantial impact on TBT. This can involve techniques like code splitting, removing unused code, and deferring non-critical JavaScript.

Additionally, using web workers to offload tasks to a background thread can also help reduce the load on the main thread and improve TBT.


Understanding and optimizing Total Blocking Time is essential for providing a smooth and responsive user experience. By measuring TBT and taking steps to minimize long tasks, especially those related to JavaScript, you can ensure that your webpage is always ready to respond to user interactions promptly.

Remember, a lower TBT not only leads to happier users but also contributes to better SEO rankings. So, don’t overlook this important performance metric!

Leave a Comment

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

Click to access the login or register cheese