INP (Interaction to Next Paint)

INP (Interaction to Next Paint) is a Core Web Vitals metric that measures how quickly a site visually responds to a user interaction - a click, a tap on a touchscreen, or a key press. In March 2024 it replaced the older FID (First Input Delay) metric and has since been one of the three main indicators Google uses to judge user experience, which in turn affect search rankings.

The difference from FID is significant. FID measured only the delay on the very first interaction, and only up to the moment the browser started processing it. INP tracks every interaction during a visit and measures the whole time from the user's action to the visual response being painted. It's a far more honest picture of how responsive a site feels during real use, not just at first contact.

The thresholds work like this: 200 milliseconds or under is good, 200 to 500 ms needs improvement, and over 500 ms is poor. It's assessed at the 75th percentile of real visits, so it isn't enough for a site to respond quickly 'usually' - it has to stay snappy on slower devices and worse connections too.

The most common cause of poor INP is heavy JavaScript blocking the browser's main thread. When a user clicks and the browser is busy processing a big chunk of script, the response is delayed. It helps to break long tasks into smaller ones, defer unnecessary JavaScript, do less work during rendering, and generally be restrained about how much code you ship to a page. INP is a useful wake-up call here: it rewards sites that aren't overloaded.