Interaction to Next Paint (INP) is the responsiveness metric that replaced First Input Delay (FID) in the Core Web Vitals set in March 2024. INP measures the latency between a user input (tap, click, keypress) and the next visual update — capturing the page's responsiveness over its entire lifetime, not just the first interaction (which FID measured).
The 2026 thresholds are: under 200ms = good, 200-500ms = needs improvement, over 500ms = poor. INP is more demanding than FID because it samples every interaction, not just the first one. Many pages that passed FID fail INP.
The most common INP problems are long JavaScript tasks blocking the main thread (especially during third-party script execution), heavy React/Vue re-renders triggered by user input, synchronous handlers that should be deferred to requestIdleCallback or web workers, and expensive layout-thrashing DOM operations inside event handlers.
For SEO, Google uses INP as a Core Web Vitals ranking signal (since the FID-to-INP transition). For AEO, INP contributes to user-experience quality signals that AI engines weigh in their source assessment. The optimization is the same: shorter main-thread tasks, deferred work, lighter event handlers.