How Can I Identify and Optimize Inefficient JavaScript Code Affecting First Input Delay (FID) and Overall Interactivity?


First Input Delay (FID) measures the time from when a user first interacts with a page to the time when the browser is able to begin processing event handlers in response to that interaction. Optimizing inefficient JavaScript code can significantly improve FID and overall interactivity. This involves identifying blocking JavaScript tasks and optimizing your JavaScript files through techniques such as code splitting, removing unused code, and deferring non-critical JavaScript.

Identify Blocking JavaScript Tasks

Long Tasks API

Use the Long Tasks API to identify tasks in JavaScript that block the main thread over 50ms. These tasks are often the culprit behind high FID scores [Long Tasks in Devtools, 2022].

Optimize Your JavaScript Files

Code Splitting

Implement code splitting to only load the JavaScript necessary for the current view, reducing the amount of code that needs to be compiled and executed [Reduce JavaScript Payloads with Code Splitting, 2022].

Remove Unused Code

Eliminate unused JavaScript using tools like PurgeCSS and UnCSS. This cuts down code volume and helps improve JavaScript parse and compile times [Remove Unused Code, 2022].

Defer Non-Critical JavaScript

Use the `defer` or `async` attribute when including JavaScript files. The `defer` attribute allows you to keep the scripts in the HTML and does not block rendering [Efficiently Load Third-Party JavaScript, 2022].

Use Web Workers

Web Workers allow you to run JavaScript on background threads, offloading complex computations and keeping the main thread available for user inputs, a crucial aspect of optimizing FID [Use Web Workers for Your Computations, 2022].


To optimize inefficient JavaScript code affecting First Input Delay, the best practices include identifying and minimizing long JavaScript tasks, optimizing your JavaScript files using techniques like code splitting and removing unused code, deferring non-critical JavaScript, and leveraging Web Workers to run complex computations off the main thread.