Max Potential First Input Delay (FID)

Max Potential First Input Delay (FID) is one of six metrics tracked in the Performance section of the Lighthouse report. Each metric captures some aspect of page load speed. Lighthouse displays Max Potential FID in milliseconds.

Table of Contents

  1. What does Max Potential FID measure?
  2. How can I improve my Max Potential FID score?
  3. Further Information

Max Potential First Input Delay example of Zeit.de The homepage of Zeit.de takes about one second to respond to user input, according to the FID.

What does Max Potential FID measure?

Max Potential FID is an estimate of how long your app takes to respond to user input during the busiest 5 seconds of page load. FID measures the time from when a user first interacts with your site to the time when the browser responds to that interaction. In general, input delay happens because the browser's main thread is busy doing something else—like parsing a large JavaScript file—that keeps it from responding to the user.

FID focuses on discrete user actions like clicks, taps, and key presses. The performance of continuous actions, like scrolling and zooming, are affected by different factors and so should be evaluated separately.

FID can vary depending on when a user interacts with your page; if the user happens to start an interaction when the browser's main thread isn't busy, there won't be a delay. That's why Lighthouse measures the maximum potential FID by checking for input delay during the main thread's busiest window. This approach helps provide a more reliable metric that you can use to evaluate the responsiveness of your page over time.

How can I improve my Max Potential FID score?

The strategies for improving Max Potential FID are largely the same as the strategies for improving Time to Interactive or other performance metrics. Look for opportunities to optimize your JavaScript. In particular, consider reducing JavaScript payloads with code splitting and applying the PRPL pattern. Optimizing third-party JavaScript also yields significant improvements for some sites.

Unless you have a specific reason for focusing on a particular metric, it's usually better to focus on improving your overall Performance score. Use the Opportunities section of your Lighthouse report to determine which improvements will have the most value for your page. The more significant the opportunity, the greater the effect it will have on your Performance score.

Max Potential First Input Delay example of Bahn.de

Before tackling the FID, Bahn.de should improve the Time to Interactive which takes 12 seconds.

Further information

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