What is First Input Delay (FID)?

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Ravi Vishwakarma · Answer

First Input Delay (FID) is a web performance metric that measures the time from when a user first interacts with your site (like clicking a link, tapping a button, or using a custom JavaScript-powered control) to the time when the browser is actually able to begin processing that interaction. Key Points: FID measures delay — not the total time it takes to process the event, just the delay before the event handler starts executing.It captures real-user interaction latency — only the first interaction counts.FID is important because it reflects the site’s responsiveness from the user’s perspective. Why FID Happens: The most common cause of high FID is the browser's main thread being blocked — often by long JavaScript tasks — preventing it from responding to user input. Ideal FID: Good: &lt; 100 millisecondsNeeds Improvement: 100–300 millisecondsPoor: &gt; 300 milliseconds Example: If a user clicks a button, but the main thread is busy running a large script and can’t handle the click until 400ms later, the FID would be 400ms. Note: Google is replacing FID with Interaction to Next Paint (INP) as a more comprehensive responsiveness metric in March 2024, since INP reflects the responsiveness of all interactions, not just the first.

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What is First Input Delay (FID)?

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Key Points:

Why FID Happens:

Ideal FID:

Example:

Note: