What Strategies Can I Employ to Reduce the Impact of Third-Party Scripts on First Input Delay (FID)?


First Input Delay (FID) measures the time from when a user first interacts with a page to the time when the browser is actually able to respond to that interaction. Reducing the impact of third-party scripts on FID involves optimizing script loading and execution, deferring non-critical scripts, and using modern web performance techniques. Here’s a comprehensive guide to improving FID by effectively managing third-party scripts.

Script Loading Optimization

Asynchronous Loading

Use the <script async> attribute to load third-party scripts asynchronously. This allows the browser to continue parsing HTML and fetching other resources while the script is being downloaded in the background.


<script async src="https://example.com/third-party-script.js"></script>

The async attribute is particularly useful for scripts that are not essential for the initial rendering of the page [Optimize LCP, 2023].

Defer Non-critical Scripts

Use the <script defer> attribute to delay the execution of scripts until the HTML document has been fully parsed. This can help improve FID by ensuring that the browser is not blocked by script execution during the initial load.


<script defer src="https://example.com/non-critical-script.js"></script>

The defer attribute is ideal for scripts that are not necessary for the immediate interactivity of the page [Defer Non-Critical JavaScript, 2023].

Code Splitting and Lazy Loading

Code Splitting

Code splitting allows you to break down your JavaScript into smaller chunks that can be loaded on demand. By doing so, you reduce the amount of JavaScript that needs to be loaded and executed initially, which can significantly improve FID.

Example with Webpack:

import(/* webpackChunkName: "vendors" */ 'vendor').then(module => {
// Use vendor module

This technique ensures that only the essential code is loaded initially, while less critical parts are loaded as needed [Reduce JavaScript Payloads with Code Splitting, 2023].

Lazy Loading

Lazy load third-party scripts that are not essential for the initial page load. This can be achieved using Intersection Observer API or libraries like lozad.js to load resources only when they are needed.


const observer = new IntersectionObserver((entries) => {
entries.forEach(entry => {
if (entry.isIntersecting) {
const script = document.createElement('script');
script.src = 'https://example.com/third-party-script.js';

This helps in deferring the load of non-essential scripts until they are required, improving FID [Lazy Loading, 2023].

Minimize JavaScript Execution Time

Optimize Heavy Scripts

Identify and optimize heavy third-party scripts that may be causing significant delays. Use tools like Chrome DevTools to analyze script performance and refactor or defer them if possible [Bootup Time, 2023].

Reduce Script Size

Minify and compress third-party scripts to reduce their size. Tools like UglifyJS, Terser, and compression algorithms such as Gzip and Brotli can be used to achieve smaller script sizes.

Example with UglifyJS:

uglifyjs yourscript.js -o yourscript.min.js -c -m

This helps in speeding up download times and reducing the impact on FID [Make Your JavaScript Fast, 2023].

Use Web Workers

Offload Tasks to Web Workers

Web Workers allow you to run scripts in background threads. This can help offload heavy or long-running tasks from the main thread, thus improving FID.


const worker = new Worker('worker.js');
worker.onmessage = function(event) {
console.log('Worker result:', event.data);

By moving complex computations to Web Workers, the main thread remains free to respond to user interactions promptly [Offload Tasks to a Web Worker, 2023].


Improving FID by reducing the impact of third-party scripts requires a multifaceted approach that includes optimizing how scripts are loaded and executed, deferring non-critical scripts, lazy loading, and using modern performance techniques like Web Workers. Implementing these strategies will help ensure a faster and more responsive web experience for users.