Why Interaction to Next Paint Changed How Developers Think About Responsiveness

For years, First Input Delay was the responsiveness metric in Core Web Vitals. It measured one thing: the delay between a user's first interaction and the browser starting to process it. Most sites passed FID easily because it only captured input delay on the first click, ignoring everything after that. A page could freeze for two seconds when you opened a dropdown menu, and FID would not register it. In March 2024, Google replaced FID with Interaction to Next Paint. The shift was not cosmetic. INP fundamentally changed what "responsive" means for web applications by measuring every interaction across the entire page visit, not just the first one, and by tracking the complete lifecycle of each interaction through input delay, processing, and paint. This article explains why the change matters, what INP actually measures, and how to think about responsiveness now that the rules have changed. What FID Got Wrong First Input Delay measured the gap between a user's first click (or tap or ke