First Input Delay (FID)

[00:00:00]
>> Todd Gardner: This is not a Core Web Vital anymore, but I wanna talk about it in memorial. Up until recently, the third Core Web Vital used to be the First Input Delay or FID. And you're gonna still see a lot of content about this online because this is a relatively recent change as of 2024.

[00:00:19]
When the Core Web Vitals first launched in 2020, FID was the third one, and it was retired in 2024 and replaced with INP because it's a better measure for user interactivity. But let's just briefly talk about the first input delay and what it is. Essentially, this measured the first INP, this was measuring the first time the user interacts.

[00:00:43]
How long does it take in order to like measure stuff? Now, why was that important? The first input delay was largely measuring the time right up here, it was measuring the time at the very beginning of that page load sequence. Because the first time the user interacts is the highest likelihood that stuff was still working, that JavaScript was still running, stuff was being compiled, images were being downloaded, and the main thread would generally be busy.

[00:01:15]
And so the first input delay was trying to measure are you sending an obnoxious amount of JavaScript and other resources down to the page, and is it getting in the way of the user trying to interact? Now it was replaced because it generally didn't measure things super effectively.

[00:01:33]
It really emphasized that blocking time over processing time. And a lot of people have very inefficient JavaScript that spends a lot of time processing it, and users will have lots and lots of interactions on your page. Hopefully, hopefully, a user stays a long time and does a lot of good stuff.

[00:01:53]
And so we want to measure all of them. A site could still feel slow if the third or fourth or 100th interaction is the important one that they click on and the site feels laggy at that point. So FID didn't measure those things, and so that's why it was retired very recently and replaced with interaction to next paint is a better measure for user interactivity.