First Input Delay (FID) was a web performance metric that measured how long a page made a visitor wait the first time they tried to interact with it. Think of a click on a button, a tap on a link or a keypress in a form. FID counted only that first action, and only the delay before the browser began to respond, not how long the response took to finish.
Picture walking up to a shop counter and saying hello, but the clerk is mid-phone-call and takes five seconds to look up. That pause is FID. The browser was busy with something else (usually running JavaScript) and could not turn its attention to you yet. A short pause feels instant. A long one feels broken.
FID was one of the three Core Web Vitals for years, and tools like Lighthouse reported on it. In March 2024 Google retired it and put Interaction to Next Paint in its place, because INP measures every interaction across a visit, not just the opening one. So FID is now mostly a historical term, but the underlying lesson stands: a page can look finished while still being slow to respond.
One quirk is worth knowing if you read old reports. FID could only be measured from real visitors, since it needs an actual first interaction to happen, so lab tools like Lighthouse reported a stand-in called Total Blocking Time instead. That is why a page might score well in a lab test yet feel sluggish to a real user on a cheap phone, where the JavaScript takes far longer to chew through.
At TopDevs we keep interaction delays low by shipping less JavaScript and loading what is left at the right moment, so the first click always lands.
