Discussion g/technical-seo 23d ago

@carlos_geo

Interaction to Next Paint (INP) officially replaced First Input Delay (FID) as a Core Web Vitals metric in March 2024. I have been auditing sites since then and the majority of 'passing' sites I see are actually failing INP on mobile. **Why INP is harder to pass than FID:** FID only measured the first interaction. INP measures *every* interaction throughout the page session and reports the worst one. A single laggy dropdown or accordion can fail your entire page. **Common culprits I keep finding:** 1. **Third-party scripts** (chat widgets, analytics, ad scripts) running on the main thread during interaction 2. **Large JavaScript bundles** parsed synchronously on load 3. **CSS animations** using properties that trigger layout recalculation (top/left instead of transform) 4. **Unoptimised event listeners** firing on every scroll or keystroke **How to diagnose:** - Chrome DevTools > Performance panel > record while interacting with the page - PageSpeed Insights > 'Diagnose performance issues' section - Web Vitals Chrome extension shows real-time INP while you browse **Quick wins:** - Defer all non-critical scripts with `async` or `defer` - Move heavy event handlers off the main thread using Web Workers - Replace JS-powered accordions with native `<details>` / `<summary>` HTML elements Good threshold: under 200ms. Needs improvement: 200–500ms. Poor: over 500ms.

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