Interaction to Next Paint replaced First Input Delay, and plenty of sites that passed comfortably before suddenly failed.
What changed
FID only measured the delay before the first interaction was handled. INP measures how long every interaction takes to paint a response, so slow click handlers that FID never saw are now counted.
The usual culprits
Across 4,000 domains, heavy third-party scripts, long tasks inside click handlers and hydration work that blocked the main thread explained most failures.
Takeaways
Break long tasks up, defer non-critical scripts, and profile the interactions people actually use, like menus, filters and add-to-cart buttons.
