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Improving Total Blocking Time: A Comprehensive Guide

Understanding and improving Total Blocking Time (TBT) is crucial for enhancing your website’s performance. TBT is one of the key metrics used to measure the responsiveness of a website. It quantifies the total amount of time that a page is blocked, preventing users from interacting with it. This guide will provide you with in-depth knowledge on TBT and practical tips on how to improve it.

Understanding Total Blocking Time

Before we delve into the ways to improve TBT, it’s important to understand what it is and why it matters. Total Blocking Time measures the total time between First Contentful Paint (FCP) and Time to Interactive (TTI) where the main thread was blocked for long enough to prevent input responsiveness.

Long tasks, those that take more than 50 milliseconds, contribute to high TBT. These tasks could be anything from heavy JavaScript parsing and execution, to large style calculations, and more. When the main thread is blocked by these tasks, users can’t interact with the page, leading to a poor user experience.

Why Total Blocking Time Matters

Total Blocking Time is a crucial metric because it directly impacts the user experience. A high TBT means that users have to wait for the page to become interactive, which can be frustrating and lead to users leaving the site.

Moreover, TBT is a part of Google’s Core Web Vitals, a set of metrics that Google considers important in a webpage’s overall user experience. Hence, improving your TBT can lead to better SEO rankings.

How to Improve Total Blocking Time

Improving TBT involves reducing the impact of long tasks, optimizing your code, and leveraging browser capabilities. Here are some strategies that can help:

Minimize Long Tasks

Long tasks are the primary contributors to high TBT. By breaking them down into smaller, asynchronous tasks, you can ensure that the main thread is not blocked for extended periods.

For instance, if you have a large JavaScript file that is parsed and executed as a single block, consider splitting it into smaller chunks. This way, the browser can incrementally work on the tasks without blocking the main thread.

Optimize Your JavaScript

JavaScript often contributes to high TBT because it blocks the main thread when parsing and executing. Optimizing your JavaScript can significantly improve your TBT.

Some strategies include deferring non-critical JavaScript, minifying your JavaScript files, and using Web Workers to run scripts in the background without blocking the main thread.

Leverage Browser Caching

Browsers can cache a significant amount of information, which can help improve TBT. When a user visits your site, the browser can store static files like CSS, JavaScript, and images. When the user revisits your site, the browser can load these files from the cache instead of downloading them again.

This reduces the amount of data that needs to be processed, thereby reducing TBT. Ensure that your server is configured to deliver the appropriate caching headers for these static resources.

Advanced Techniques for Improving Total Blocking Time

While the strategies above can significantly improve your TBT, there are advanced techniques that can provide further enhancements. These include using a Content Delivery Network (CDN), implementing server-side rendering, and using a service worker.

Use a Content Delivery Network (CDN)

A CDN can help reduce TBT by delivering your content from servers close to your users. This reduces the time it takes for data to travel from your server to the user’s browser, which can improve TBT.

Moreover, CDNs can cache your content, further reducing the amount of data that needs to be processed by the browser.

Implement Server-Side Rendering (SSR)

SSR can improve TBT by reducing the amount of JavaScript that needs to be processed on the client-side. With SSR, your server generates the full HTML for a page in response to a request. This means that the browser can start rendering the page sooner, improving TBT.

However, SSR can increase server load, so it’s important to implement it carefully and monitor your server’s performance.

Use a Service Worker

Service workers can help improve TBT by offloading some tasks from the main thread. They run in the background, separate from the main browser thread, and can handle tasks like caching, background sync, and push notifications.

By offloading these tasks to a service worker, you can reduce the load on the main thread, improving TBT.


Improving Total Blocking Time is a multifaceted process that involves understanding how your website’s code impacts the main thread and implementing strategies to minimize this impact. By optimizing your JavaScript, leveraging browser caching, and using advanced techniques like CDNs, SSR, and service workers, you can significantly improve your TBT and enhance your website’s performance.

Remember, a better TBT not only improves user experience but also contributes to better SEO rankings. So, start implementing these strategies today and see the difference they can make to your website’s performance.

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