What is First Input Delay (FID)?

Question

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: andlt; 100 millisecondsNeeds Improvement: 100–300 millisecondsPoor: andgt; 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)?

Anubhav Kumar

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Key Points:

Why FID Happens:

Ideal FID:

Example:

Note:

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