First Input Delay

First Input Delay (FID) is a time based metric that describes JavaScript main thread activity. It is useful for understanding how long a page is unable to respond to user input.

How is First Input Delay calculated?

First Input Delay describes the time between user interacting with a page (including clicks, taps, and key presses) and the time when the browser can process the event handlers in response to that interaction. Essentially, it measures the latency between user action and browser response, which might be delayed by a busy JavaScript main thread.

First Input Delay depends on real user input, so it’s available in real user monitoring (field) tools and reports such as Chrome User Experience Report (CrUX). Calibre doesn’t report First Input Delay at this time, but Total Blocking Time (TBT) is a good synthetic monitoring alternative.

What First Input Delay measurement is fast?

Pages with < 100 ms of input delay can be considered as fast and durations over 300 ms are considered slow.