FID (First Input Delay)

FID (First Input Delay) was a performance metric that measured the time between a user's first action on a page — clicking a button, tapping a link or selecting a form field — and the moment the browser actually began processing that interaction. FID did not measure the time to run the event handler, only the delay before the response could start. Until March 2024 it was one of the three Core Web Vitals, after which the more comprehensive INP metric took its place.

How FID worked

When a user interacted with a page, the browser needed a free main thread to run the code attached to that event. If the thread was busy at the time executing a long JavaScript task, the response had to wait — and it was exactly this waiting time that FID measured. The metric relied solely on field data (real user monitoring), because it required a genuine human interaction.

Why it was replaced by INP

FID considered only the first interaction and only the delay before it started, which often gave an overly optimistic picture. INP covers all interactions during a session and measures the full time until the response is rendered, so it reflects perceived responsiveness far better.

Practical applications

Although FID is no longer an official metric, understanding it helps make sense of how page responsiveness has been measured and how to read older SEO reports. The optimisation methods remain valid for INP: splitting long JavaScript tasks, limiting third-party scripts, deferring less important code and keeping the main thread free for user interactions. Analysts and developers compare FID against the LCP and CLS metrics to judge the overall quality of the on-page experience.