What is FID (First Input Delay)?

FID (First Input Delay) is a Core Web Vitals metric that measures the delay between a user's first action on a page (click, tap, key press) and the moment the browser actually starts reacting to it.

In simpler terms: FID shows whether the user feels "lag" during the first interaction.

If a person clicks a button and the page "freezes" for a fraction of a second, that's a bad FID.

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#What FID actually measures

• not the page's overall speed,
• not the loading speed,
• but the interface's first reaction to a user's action.

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#Google's norms

• Good: < 100 ms
• Needs improvement: 100-300 ms
• Poor: > 300 ms

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#Why a bad FID happens

The main culprit is heavy JavaScript, which:

• runs for too long,
• blocks the browser's main thread,
• prevents the page from reacting quickly to a click.

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#Why this metric matters

FID reflects the feeling of a site's "responsiveness." Even if a page loaded fast, but a button doesn't react, the user feels irritated, and this worsens behavior, conversions, and SEO.

FID is now gradually being replaced by the more precise INP (Interaction to Next Paint) metric, which measures responsiveness not only of the first, but also of subsequent interactions.