Interaction to Next Paint (INP) measures how quickly a page responds to user input across all interactions and represents interactivity.Open glossary entry → is one of the Core Web Vitals are a set of Google metrics that measure real-world user experience in the areas of loading performance, interactivity and visual stability.Open glossary entry → and measures how quickly a page responds to user interactions such as clicks or input. Unlike the older First Input Delay (FID) metric, which only looked at the first interaction, INP evaluates responsiveness across all interactions on a page. A good INP value is no more than 200 milliseconds.

A poor INP is often caused by too much JavaScript, long main-thread tasks, heavy frameworks, many third-party scripts or complex event handlers. Helpful measures include:

  • reducing JavaScript and using code splitting,
  • deferring non-critical scripts,
  • breaking up long tasks and optimising event handlers,
  • reducing Hydration is the process of making server-delivered HTML interactive in the browser via JavaScript. The markup already exists; JavaScript only adds the behaviour.Open glossary entry → and reviewing third-party scripts.

For JavaScript-heavy websites, INP is often the most important performance lever.