How Can You Use the Diagnostic Data From PageSpeed Insights to Make Targeted Improvements in Website Coding, Such as Minimizing Main-Thread Work or Reducing JavaScript Execution Time?


PageSpeed Insights provides diagnostic data that can be used to make targeted improvements to website coding. Key areas such as minimizing main-thread work and reducing JavaScript execution time can be improved by optimizing JavaScript, using efficient coding practices, and employing modern performance optimization techniques. Below is a detailed guide on how to use this data to improve your website's performance.

Minimizing Main-Thread Work

Main-thread work includes rendering, layout, and handling user interactions. To minimize this, you should focus on minimizing JavaScript and employing web workers.

Optimize and Minify JavaScript

Large and inefficient JavaScript files increase main-thread work time. Minimize and optimize JavaScript files to reduce their size:

  • Minify JavaScript to remove unnecessary characters and white spaces. Tools like UglifyJS and Google Closure Compiler can be used.
  • Use tree-shaking to remove unused code if you're using a module bundler like Webpack.

Web Workers

Offload heavy computations to web workers. Web workers run scripts in background threads, allowing the main thread to remain free for critical tasks:

const worker = new Worker('worker.js');
worker.postMessage('data'); // Send data to worker
worker.onmessage = function(event) {
  console.log(; // Handle response from worker

Learn more about web workers here.

Reducing JavaScript Execution Time

JavaScript execution time can be reduced by minimizing the amount of JavaScript that needs to be parsed and executed on load.

Techniques for Reducing JavaScript Execution Time

Code Splitting

Code splitting is a technique where code is divided into smaller bundles that are loaded on demand:

import('module').then(module => {
  // Use the module here

This ensures that unnecessary code is not loaded until needed. Learn more about code splitting here.

Lazy Loading

Lazy loading helps in loading JavaScript files only when they are required:

document.addEventListener('DOMContentLoaded', function() {
  let script = document.createElement('script');
  script.src = 'your-script.js';

This deferred loading enhances initial load performance. More about lazy loading can be found here.

Defer Non-Essential JavaScript

Use the <script> tag's async and defer attributes to ensure non-essential JavaScript loads asynchronously:

<script src="script.js" defer></script>
<script src="analytics.js" async></script>

More about deferring JavaScript can be found here

Example Implementations

Consider the following examples of JavaScript optimizations:

Throttling and Debouncing

If your site has scroll or resize event listeners, ensure you are using throttling or debouncing to limit the rate of function calls.

// Debounce function
function debounce(func, wait) {
  let timeout;
  return function executedFunction(...args) {
    const later = () => {
    timeout = setTimeout(later, wait);

More about these techniques can be found here.


By leveraging PageSpeed Insights diagnostics, you can make specific improvements to optimize main-thread work and reduce JavaScript execution time. Adopting practices such as minification, code splitting, lazy loading, and using web workers not only helps in decreasing load times but also improves overall user experience.