What is FID (First Input Delay)?

FID (First Input Delay) is a metric that measures how quickly a site responds to the user's first action.

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#What counts as an "action":

• clicking a button;
• clicking a link;
• typing into a form field.

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#What is measured:

The delay between the moment the user pressed something and when the browser started processing it. Not when the result appeared, but when the site started reacting.

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#Why this matters:

If a person clicks and nothing happens, they perceive the site as "sluggish." Even if everything is visually loaded.

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#Norms:

• up to 100 ms - good;
• 100-300 ms - acceptable;
• more than 300 ms - causes irritation.

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FID matters for interactivity. Fast loading is one thing, and a fast reaction to actions is something else entirely.