What Tools Are Best for Measuring First Input Delay (FID), and How Should I Interpret the Results to Optimize Web Performance?


First Input Delay (FID) is a crucial web performance metric that measures the time from when a user first interacts with a page to the time when the browser is actually able to begin processing event handlers. Effective tools for measuring FID include Lighthouse, Chrome User Experience Report (CrUX), and the Web Vitals extension. To optimize web performance based on FID results, it is essential to minimize main thread work, defer non-critical scripts, optimize JavaScript execution, and implement appropriate web workers. Here’s a comprehensive guide to measure and improve FID.

Tools for Measuring First Input Delay (FID)


Lighthouse is an automated tool for improving the performance, quality, and correctness of your web apps. It comes built into Chrome DevTools and can measure various performance metrics, including FID.

To measure FID with Lighthouse:

  1. Open Chrome DevTools by pressing F12 or right-clicking on the page, then selecting "Inspect".
  2. Go to the "Lighthouse" tab.
  3. Click "Generate report."

[Lighthouse, 2023]

Chrome User Experience Report (CrUX)

The Chrome User Experience Report provides real user experience data for how Chrome users experience popular destinations on the web. You can use this data to understand FID and other performance metrics based on real user interactions.

[CrUX, 2023]

Web Vitals Extension

The Web Vitals Chrome extension measures the core web vitals metrics, including FID, in real-time as you browse the web.

To use the extension:

  1. Install the Web Vitals extension from the Chrome Web Store.
  2. Navigate to any webpage, and the extension will display FID along with other core metrics.

[Web Vitals Extension, 2023]

Interpreting First Input Delay (FID) Results

FID measures the time from when a user first interacts with your site to the time when the browser is able to respond to that interaction. Here are the thresholds for interpreting FID results:

  • Good: FID < 100ms
  • Needs Improvement: 100ms ≤ FID < 300ms
  • Poor: FID ≥ 300ms

[Understanding FID, 2023]

Optimizing Web Performance Based on FID Results

Minimize Main Thread Work

Long tasks on the main thread can block the handling of user interactions, increasing FID.

Strategies include:

  • Code Splitting: Break up large JavaScript bundles into smaller, more manageable parts.
  • Optimize CSS: Minify CSS and use media queries to reduce unnecessary style recalculations.

[Optimize LCP, 2023]

Defer Non-Critical Scripts

Deferring non-critical JavaScript can help prioritize more crucial interactions.

Use the <script defer> attribute to load non-essential scripts after the main content has been loaded.

[Defer Non-Critical JavaScript, 2023]

Optimize JavaScript Execution

Efficient and optimized JavaScript execution can reduce the work required to manage user interactions.

Focus on:

  • Reducing JavaScript Payloads: Minify and compress JavaScript files.
  • Code Refinement: Eliminate unnecessary or redundant code.

[Reduce JavaScript Payloads, 2023]

Use Web Workers

Web Workers can run scripts in background threads, offloading resource-intensive tasks from the main thread.

Implement Web Workers for tasks like data processing and calculations to ensure smoother main-thread operations.

[Using Web Workers, 2023]


Effectively measuring and optimizing First Input Delay (FID) is vital for delivering a responsive user experience. By using tools like Lighthouse, CrUX, and the Web Vitals extension, and implementing best practices to minimize main thread work, defer non-critical scripts, optimize JavaScript execution, and utilize Web Workers, you can significantly enhance your webpage’s performance.