How Can I Identify and Optimize Long-Running JavaScript Tasks to Reduce First Input Delay (FID) on My Website?


Identifying and optimizing long-running JavaScript tasks is critical for reducing First Input Delay (FID) on your website. This involves using performance diagnostics tools to spot inefficient codes, breaking up long tasks, deferring and minimizing JavaScript use, and using web workers for offthread computation. Following this guide will help you analyze and remedy issues causing extended FID times.

Identifying Long-Running JavaScript Tasks

Use Performance Diagnostics Tools

Tools such as Google Chrome's DevTools, Lighthouse, and Search Console's Core Web Vitals report can help you measure FID and identify long-running JavaScript tasks that may be causing problems [RAIL Performance Model, 2020]. Look for tasks that run for more than 50ms as these will likely contribute to high FID times.

Optimizing Long-Running Tasks

Break Up Long Tasks

If you find tasks that run for more than 50ms, consider breaking them up into smaller, asynchronous tasks that can run without blocking the main thread [Long Tasks and DevTools, 2021].

Defer JavaScript

Use the `defer` attribute to delay execution of JavaScript files until the HTML parser has finished building the DOM [Efficiently load third-party JavaScript, 2022].

Minimize JavaScript

Keeping your JavaScript files as small as possible can also help reduce the time your site needs to execute them. Minify and compress your JavaScript files, and remove any scripts that aren't needed [Optimize Content Efficiency, 2021].

Use Web Workers

Web Workers allow you to run JavaScript on a background thread, which can help free up the main thread to respond to user input more quickly [Using Web Workers, 2021].


Optimizing long-running JavaScript tasks is a crucial aspect of improving your website's FID and, in turn, the overall user experience. By using performance diagnostics tools, breaking up long tasks, and utilizing strategies such as deferring JavaScript or using web workers, you can significantly enhance your site's interactivity and responsiveness.