Full Definition
Interaction to Next Paint (INP) replaced First Input Delay (FID) as a Core Web Vital in March 2024. It measures the responsiveness of a page across its entire lifespan — not just the first interaction (as FID did), but every click, tap, or keyboard press throughout the user's visit. INP captures the worst interaction delay observed (excluding outliers), measured in milliseconds: - **Good**: Below 200ms - **Needs improvement**: 200–500ms - **Poor**: Above 500ms At above 200ms, users perceive a noticeable lag between their action and the page's response — a button that feels 'slow' or a form field that takes a beat to respond. At 500ms, many users will assume the page is broken and abandon. The primary causes of poor INP: **Heavy JavaScript execution**: When the browser's main thread is busy running JavaScript, it can't respond to user interactions. Large JavaScript bundles, excessive third-party scripts (chat widgets, analytics, heatmaps), and unoptimised event handlers are the main culprits. **Long tasks**: Any JavaScript task that runs for more than 50ms is called a 'long task' and blocks the main thread. Break these into smaller chunks using `requestAnimationFrame` or `scheduler.yield()`. **DOM complexity**: Extremely large, deeply nested HTML trees slow down rendering updates triggered by user actions. For most business websites built on WordPress or standard CMS platforms, the biggest INP wins come from reducing third-party script count and deferring non-critical JavaScript. Actionable tip: In Chrome DevTools, go to the Performance tab and record a session where you interact with your page. Long yellow bars in the Main thread view represent long tasks — these are your INP culprits.