Chapter 5: First Input Delay (FID)

First Input Delay (FID) measures the time from when a user first interacts with a page to when the browser can respond to that interaction.

In other words, FID aims to capture load responsiveness (interactivity) or how fast a page responds to user interaction.

FID is evolving, but its current definition splits out three critical concepts to understand.

FID only measures discrete events as user interaction, such as clicks, toggling, taps, and key presses. Events that FID measures as user interaction are part of the Response aspect of the RAIL model, precisely the following:

  • click
  • keydown
  • mousedown
  • pointerdown (only if it is followed by pointerup)

Other types of user interaction, like zooming or scrolling, won’t impact FID.

FID only considers the first input because:

  • The first time a user interacts with your page will shape the user’s first impression of your page responsiveness.
  • The most critical interactivity issues occur during page load.
  • Slow input delays after page load may require different solutions.

FID measures input delay, not processing. Thus, FID measures the time the browser takes to respond to the user interaction event, not the event processing time itself. The diagram below illustrates this idea.

Source: Google