How Can Breaking Up Long JavaScript Tasks Into Smaller Chunks Improve First Input Delay (FID) on Complex Pages?


Breaking up long JavaScript tasks into smaller chunks significantly enhances First Input Delay (FID) on complex web pages. This technique helps ensure that the main thread remains responsive, enabling faster user interaction. The following detailed breakdown explains how this approach works, provides specific strategies, and offers examples to improve FID effectively.

Understanding First Input Delay (FID)

First Input Delay (FID) is a web performance metric that measures the delay between a user's first interaction with a page (such as clicking a link or tapping a button) and the moment the browser is able to respond to that interaction. A low FID is critical for ensuring a smooth user experience, particularly on interactive pages.

Why Main Thread Blocking is Problematic

JavaScript executes on the browser's main thread. When a JavaScript task is too long, it can block the main thread, preventing the browser from processing user interactions promptly. This blockage results in a higher FID, leading to a frustrating experience for users.

Breaking Up Long JavaScript Tasks

Task Chunking Concept

Task chunking involves dividing larger JavaScript tasks into smaller, more manageable pieces. By distributing work into smaller chunks, the browser can handle user interactions more efficiently, as shorter tasks allow the main thread to remain responsive.

Practical Approaches and Techniques

Using requestIdleCallback()

The requestIdleCallback() method allows you to perform background and low-priority work on the main thread without impacting user interactions. Tasks scheduled with requestIdleCallback() are executed during the browser's idle periods.


function performTask() {
  // Long task code

More on requestIdleCallback(): MDN Web Docs

Using setTimeout() and setInterval()

Breaking tasks using setTimeout() and setInterval() can spread work over multiple event loop cycles, thereby keeping the main thread free for user interactions.


function performChunkedTask(tasks) {
  if (tasks.length === 0) return;
  tasks.shift()(); // Execute a task
  setTimeout(() => performChunkedTask(tasks), 10); // Schedule the next chunk
const tasks = [task1, task2, task3]; // Define task functions

More on using timers: MDN Web Docs

Using Web Workers

Web Workers allow you to run scripts in background threads. They execute independently of the main thread, which helps prevent blocking and can improve FID performance on complex pages.


// main.js
const worker = new Worker('worker.js');
worker.onmessage = (e) => console.log(;

// worker.js
onmessage = (e) => {
  if ( === 'start') {
    // Long-running task 
    postMessage('Task complete'); 

More on Web Workers: MDN Web Docs

Case Study and Example

Example: Infinite Scrolling Content

In an infinite scrolling application, loading all content at once can significantly delay user interactions. By chunking tasks such as fetching and rendering content, you improve FID:

function loadContent() {
  // Simulate a long task by splitting into chunks
  const contents = [...Array(1000).keys()];
  function fetchChunk(chunk) {
    return new Promise((resolve) => setTimeout(() => resolve(chunk), 50));
  async function loadChunks(chunks) {
    for (const chunk of chunks) {
      const data = await fetchChunk(chunk);
 function renderContent(data) {
  // Render logic

This approach keeps the main thread responsive while loading large datasets in user-friendly bursts.

Additional Optimization Techniques

Lightweight Frameworks and Libraries

Use lightweight libraries or frameworks that manage asynchronous tasks efficiently, such as React with concurrent mode or Vue.js with asynchronous components.

More on concurrent mode in React: React Concurrent Mode

Code Splitting

Implement code splitting to divide code into smaller bundles, ensuring only necessary scripts are loaded first.

More on code splitting: Webpack Documentation


Breaking up long JavaScript tasks into smaller chunks is an effective strategy to improve First Input Delay (FID) on complex pages. By ensuring the main thread is not blocked for extended periods, user interactions are handled promptly, resulting in an enhanced user experience. Implementing techniques such as requestIdleCallback(), setTimeout(), Web Workers, and code splitting can dramatically reduce FID and optimize web performance.