INP (Interaction to Next Paint) is a Core Web Vital metric that measures the latency of the slowest interaction a user makes with a page during their entire visit, covering click, tap, and keyboard events. It replaced FID (First Input Delay) as an official Core Web Vital in March 2024. Where FID only measured the delay before the browser could start processing an event, INP captures the full time from interaction to the moment the next frame is visually presented, making it a more complete measure of runtime responsiveness. Google considers a page “good” if INP is 200 milliseconds or less, “needs improvement” up to 500 milliseconds, and “poor” beyond that. Heavy JavaScript execution, long tasks on the main thread, and excessive DOM size are the primary causes of poor INP. Optimising INP typically involves breaking up long tasks, deferring non-critical JavaScript, and using web workers to move processing off the main thread.