INP observes every interaction a visitor has with your page — clicks, taps, key presses — and reports the worst one (approximately the 98th percentile). Good INP is under 200ms, meaning the browser visually responds to the interaction within 200ms. Over 500ms is poor. Google made INP an official Core Web Vital in March 2024, replacing First Input Delay.
INP is a harder metric to optimize than FID was, precisely because it measures everything, not just the first click. Your page might load perfectly and respond to the first CTA click instantly — but if opening a mobile menu takes 400ms because a heavy script runs on that interaction, your INP score tanks. It catches the real-world experience, not just the best-case scenario.
What kills INP on landing pages
Third-party scripts are the #1 offender. Analytics, A/B testing tools, chat widgets, and heatmap recorders all add JavaScript that runs on the main thread. When a visitor clicks something, the browser has to wait for these scripts to yield before it can process your click handler. The fix: audit your third-party scripts ruthlessly, defer anything that isn't critical, and use requestIdleCallback for non-essential work.
For landing pages specifically: if clicking your CTA triggers a form modal, make sure that modal's JavaScript is preloaded or code-split so it's ready before the click. A 300ms delay between "click CTA" and "see form" is the exact moment you lose conversions.