What Is First Input Delay (FID) and Interaction to Next Paint (INP)

Short answer

First Input Delay measures the delay between a user's first interaction and the browser's response. Interaction to Next Paint is its successor, measuring the latency of all interactions throughout the page lifecycle.

How it works

FID only captures the first interaction. INP observes all clicks, taps, and key presses, selecting the worst latency as the score. Both metrics reflect how busy the main thread is with JavaScript execution.

Example

A user clicks a button while the main thread is parsing a large JSON file. The browser cannot respond until parsing finishes, resulting in a high FID or INP score.

Why it matters

Responsive interfaces feel snappy. Poor interactivity scores indicate that heavy scripts block user interactions, leading to frustration.