What is FID (First Input Delay)?

FID (First Input Delay) is a metric that measures the delay between a user's first action and the browser's response.

#What counts as an "action"?

• clicking a button,
• pressing a key,
• selecting a form field. (Scrolling and zooming do not count.)

#What it looks like:

The user clicks, but nothing happens. Why? Because the browser is still busy (for example, processing heavy JavaScript). FID shows how quickly the interface can start responding.

#The benchmark:

• Good: up to 100 ms
• Acceptable: 100-300 ms
• Poor: more than 300 ms

#Why it matters:

Even if the page has visually loaded, FID shows whether you can actually interact with it yet. If a user presses a button and it stays "silent", that's bad UX.

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#Important:

As of March 2024, FID has officially been replaced by INP (Interaction to Next Paint), a more accurate responsiveness metric.

But FID is still used in older systems and materials, so knowing it remains relevant.