First Input Delay (FID): Definition and Replacement by INP

What is First Input Delay (FID)?

First Input Delay (FID) was a Core Web Vital that measured the time from a visitor's first interaction with a page, such as a click, tap or key press, until the browser could start running the event handlers for it. It captured the "frozen" feeling when JavaScript keeps the main thread busy during load and the page cannot react. A good FID was 100 milliseconds or less at the 75th percentile.

FID is no longer a Core Web Vital. Interaction to Next Paint (INP) replaced it on 12 March 2024, and web.dev states that support for FID ended on 9 September 2024. This page explains what FID was and how it differs from INP. For anything you are measuring or fixing today, use INP.

How FID worked

FID only measured the input delay of the first interaction on a page: the gap between the input and the moment its handlers could begin. It did not include the time the handlers took to run or the time to paint the result. Continuous actions such as scrolling and zooming were not part of it.

It could only be measured with real users, since it needs a real first interaction. In the lab, Total Blocking Time (TBT) was the usual stand-in, and web.dev notes that it correlated well with FID in the field.

FID versus INP

Why FID was replaced

• It saw one interaction. A page could have a good FID and still feel slow on the tenth click.
• It ignored the response. Slow event handlers and slow rendering after the input did not count.
• It was easy to pass. Visitors who interacted after the page had settled saw almost no delay, so many sites passed without feeling responsive.

What still applies

The causes of a poor FID are also causes of a poor INP, so most of the old advice carries over: long JavaScript tasks, large bundles, third-party scripts, heavy hydration and slow event handlers. Break up long tasks, load code only when needed and defer what is not urgent. Common mistakes today are still tracking FID in dashboards and alerts, and treating a "passed" FID report from before 2024 as proof that the page feels responsive.

Measuring responsiveness today

Check INP in field data from the Chrome UX Report (CrUX). The free Core Web Vitals checker reports LCP, INP and CLS from CrUX, and as its page says, it reports INP, not FID. To follow all of the Core Web Vitals over time, LoadFocus page speed monitoring runs scheduled Lighthouse audits that include Total Blocking Time. The full optimization guide is how to optimize Interaction to Next Paint, and for alerts and reports see how to monitor Core Web Vitals. The original FID definition is archived on web.dev.

FAQ: First Input Delay

Is FID still a Core Web Vital?

No. INP replaced FID as a Core Web Vital on 12 March 2024.

What was a good FID?

100 milliseconds or less at the 75th percentile of page loads.

What is the difference between FID and INP?

FID timed only the input delay of the first interaction. INP times every interaction from input to the next paint and reports the slowest one.

Can I still see FID in my tools?

Some older reports and libraries may still show it, but Google moved its tooling to INP. web.dev says FID support ended on 9 September 2024, so do not rely on it.

Does a good FID mean a good INP?

Not necessarily. A page can respond quickly to its first input and still be slow on later clicks or in event handlers, which FID never measured.