What is First Input Delay (FID)?

Question

ICSM Computer · 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.

forum

What is First Input Delay (FID)?

Can you answer this question?

1 Answers

Key Points:

Why FID Happens:

Ideal FID:

Example:

Note: