How Can I Prioritize User-Interactive Requests to Improve First Input Delay (FID) in My Web Applications?


Improving First Input Delay (FID) involves optimizing web application performance to ensure that user interactions are prioritized and processed swiftly. This comprehensive guide covers techniques like breaking up long tasks, optimizing JavaScript execution, leveraging web workers, and using modern browser APIs to enhance FID.

Breaking Up Long Tasks

Identify Long Tasks

Use performance monitoring tools to identify long-running tasks that block the main thread. Tools such as Chrome DevTools and User Timing API can help locate and measure these tasks.

Task Scheduling

Break long tasks into smaller, asynchronous tasks to ensure the main thread is regularly available for user interactions. This technique is often referred to as "RAFs and RICs" (requestAnimationFrame and requestIdleCallback), allowing tasks to yield to the browser, thus improving FID.

Optimizing JavaScript Execution

Code Splitting

Splitting your JavaScript code into smaller bundles can reduce the initial load time and ensure that only the necessary code is executed. Tools like Webpack enable code splitting effectively.

Defer and Async Scripts

Leverage the <script deferred> and <script async> attributes to prevent JavaScript from blocking the rendering of your page. Deferred scripts run in order after the HTML is parsed, while async scripts are executed as soon as they are downloaded.

Leveraging Web Workers

Offload Heavy Computation

Use Web Workers to offload heavy computational tasks from the main thread, allowing the user interface to remain responsive. Web Workers run in the background and do not block the main thread [Mozilla Developer Network, 2023].

Using Modern Browser APIs

Request Animation Frame

The requestAnimationFrame API schedules visual updates, ensuring smooth animations and interactions. This API helps to synchronize your code with the browser’s refresh rate, thus improving performance and user experience [Mozilla Developer Network, 2023].

Idle Callback

Utilize the requestIdleCallback API to run background tasks during browser idle periods, ensuring that these tasks do not interfere with the main thread’s responsiveness [Mozilla Developer Network, 2023].

Minimizing Render-Blocking Resources

Critical CSS

Inline critical CSS to ensure that essential styles are loaded immediately, thus reducing the time before the first user interaction can be processed. Tools like Critical can automate this process.

Async Loading Non-Critical CSS

Load non-critical CSS asynchronously to ensure that the main thread isn’t blocked by stylesheets that aren’t immediately needed. This can be done using the <link rel="preload" as="style"> and <link rel="stylesheet" media="print" onload="'all'"> techniques.

Examples and Best Practices

Example: Breaking Up Long Tasks

For a data-intensive operation, break the work into smaller chunks using setTimeout:

function processLargeDataSet(data) {
  const chunkSize = 1000;
  function processChunk() {
    const chunk = data.splice(0, chunkSize);
    for (let item of chunk) {
      // Process item
    if (data.length > 0) {
      setTimeout(processChunk, 0);

Example: Using Web Workers

Create a Web Worker for heavy computations:

// main.js
const worker = new Worker('worker.js');
worker.postMessage({data: largeDataSet});
worker.onmessage = function(event) {
  console.log('Processed data:',;

// worker.js
onmessage = function(event) {
  const processedData = processData(;
  function processData(data) {
    // Perform heavy computation
    return data;