If you’ve heard of Core Web Vitals and Page Experience, First Input Delay is one of the metrics associated with those.
And in this case, speed is part of a good first impression; not speed of total load time–that’s another metric, but speed as defined below.
What Is First Input Delay?
From my understanding, I’d say it’s basically how long it takes before a page (or server) starts responding to a request. In other words, if you click on a link, FID is how long it takes before a server starts sending the initial data to your browser.
It’s not the total time it takes to load a page–I believe that’s a different metric. It’s basically how long it begins to load. That’s why it’s called a delay–it’s that gap in time between a request and the beginning of its fulfillment.
What’s an Acceptable FID Time?
About 100 milliseconds or less.
Why Is FID Important?
Well, it’s part of a good first impression, and it’s part of the Page Experience concept that Google is trying to promote. A good FID helps bring about a good user experience of the page.