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First Input Delay (FID) Improvement Fix

DodaTech Updated 2026-06-24 3 min read

In this tutorial, you'll learn about First Input Delay (FID) Improvement Fix. We cover key concepts, practical examples, and best practices.

Your Lighthouse report shows a poor First Input Delay (FID) score — when a user clicks, taps, or types, the browser takes too long to respond because the main thread is busy parsing or executing JavaScript.

Step-by-Step Fix

1. Identify long tasks

Open DevTools > Performance > record interaction.

Look for Long Tasks (red triangle in the top bar) — these are tasks that take more than 50ms on the main thread.

2. Break up long tasks

// Wrong — one long task blocks the main thread
function processItems(items) {
  items.forEach(item => {
    heavyComputation(item);
  });
  // Blocks main thread for 500ms!
}

// Right — break into smaller chunks with requestIdleCallback
function processItems(items) {
  let index = 0;

  function processChunk() {
    const end = Math.min(index + 10, items.length);
    for (; index < end; index++) {
      heavyComputation(items[index]);
    }
    if (index < items.length) {
      requestIdleCallback(processChunk, { timeout: 100 });
    }
  }

  requestIdleCallback(processChunk, { timeout: 100 });
}

3. Defer non-critical JavaScript

<!-- Wrong — all scripts load synchronously -->
<head>
  <script src="analytics.js"></script>
  <script src="chat-widget.js"></script>
  <script src="app.js"></script>
</head>

<!-- Right — defer non-critical scripts -->
<head>
  <script src="app.js" defer></script>
  <!-- These load after HTML parsing -->
</head>
<body>
  <script src="analytics.js" defer></script>
  <script src="chat-widget.js" defer></script>
</body>

4. Use web workers for heavy computation

// Wrong — heavy computation on main thread
function processData() {
  const result = heavyCpuWork();
  updateUI(result);
}

// Right — offload to web worker
// worker.js
self.onmessage = function(e) {
  const result = heavyCpuWork(e.data);
  self.postMessage(result);
};

// main.js
const worker = new Worker("worker.js");
worker.postMessage(data);
worker.onmessage = function(e) {
  updateUI(e.data);
};

Common Mistakes

Mistake Fix
Loading too much JavaScript upfront Use code splitting and load on demand
No defer or async on scripts Add defer to non-critical <head> scripts
Heavy parsing on page load Defer non-critical work with requestIdleCallback
Synchronous XHR in main thread Use fetch with async/await instead
Large JavaScript bundles Analyze with webpack-bundle-analyzer and split code

Prevention

  • Keep JavaScript bundle size under 300KB (gzip compressed).
  • Use the defer attribute for all non-critical scripts.
  • Implement code splitting for route-based JavaScript.
  • Monitor FID with Real User Monitoring (RUM) tools.
  • Set a budget for JavaScript execution time in CI/CD.

DodaTech Tools

Doda Browser's interaction tracker measures FID during real user sessions and reports in its analytics dashboard. DodaZIP archives performance traces for offline FID analysis. Durga Antivirus Pro processes threat signatures off the main thread to keep UI interactions under 50ms.

Common Mistakes with input delay

  1. Using head and tail instead of pattern matching, causing runtime errors on empty lists
  2. Forgetting that lazy evaluation defers computation until the value is forced, causing space leaks with unevaluated thunks
  3. Using return to exit a function early instead of wrapping a pure value in the monad

These mistakes appear frequently in real-world FIRST code. DodaTech's contributors have identified these patterns through analysis of open-source projects and production systems.

Practice Exercise

Write a pure function that safely divides two integers using Maybe, then test it with edge cases like division by zero and negative numbers.

This exercise reinforces the concepts covered in this guide. Try implementing it before checking online solutions.

FAQ

What is a good FID score?

An FID score of 100ms or less is considered "good". 100-300ms "needs improvement", and above 300ms is "poor". FID measures the time from first user interaction to the browser processing the event handler. ||| What causes poor FID? Large JavaScript bundles, long-running synchronous tasks, complex CSS selectors, and heavy DOM operations on the main thread all contribute to poor FID. ||| How is FID different from TBT (Total Blocking Time)? FID is a field metric measured from real user interactions. TBT is a lab metric calculated from Lighthouse. They correlate — reducing TBT (by breaking up long tasks) also improves FID.

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