What is first input delay (FID)?

First Input Delay (FID) is a metric that measures how much time passes between the user's first action (for example, a click or a keypress) and the moment the browser actually starts responding to it.

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#Example

A user clicks a button right after the page loads. If the browser is busy running heavy JavaScript at that moment (for example, rendering React components), it cannot process the click right away. If the delay is 300 ms, then FID = 300 ms.

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#What counts as "input"

• a click
• a keypress
• a tap

Does not count: scrolling, zooming, hovering the mouse

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#Norms (per Google)

• < 100 ms: good
• 100-300 ms: fair

300 ms: poor

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#Why it matters

FID shows how responsive the site is in the first few seconds. A high FID means the site looks loaded but "does not listen" to the user, which is frustrating.

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#Conclusion

FID is the delay between the action and the response. The smaller it is, the more "alive" and responsive the interface feels.