In March 2024, Google replaced First Input Delay with Interaction to Next Paint as an official Core Web Vital. FID is gone. INP is what matters now — and most React apps that were passing before are failing under the new standard without anyone realizing it. What was wrong with FID FID measured how long the browser took to respond to the very first user interaction on a page. Click a button, FID measures the delay before the browser started processing that click. Just the first one. Just the start of processing, not the time until anything actually happened on screen. In practice, FID was easy to pass and bad at catching real responsiveness problems. A page could have an excellent FID score while every subsequent click — after the page was fully loaded and the user was actually using it — took 600ms to respond. FID had nothing to say about that. What INP actually measures INP measures the full interaction latency for all interactions throughout the entire page session, not just the first.…