Core Web Vitals stopped being optional a while ago
INP is the new LCP. If you are not looking at it monthly, you are guessing.
Google replaced FID with INP in March 2024. Most sites that passed Core Web Vitals before that change need to be re-assessed. FID measured the delay before an event handler ran. INP measures the full interaction duration—much harder to pass.
What INP actually measures
Interaction to Next Paint is the time from user input (click, keypress, tap) to the browser rendering the next frame. The threshold: under 200ms is Good, 200–500ms is Needs Improvement, over 500ms is Poor.
The reason most sites fail: JavaScript on the main thread. A 300ms event handler that updates the DOM will fail INP even if LCP is excellent. The culprit is usually a third-party script (analytics, chat widget, tag manager) that wasn't there when the site was last tested.
LCP: still the most important metric for rankings
Largest Contentful Paint is the time to render the largest visible element (usually a hero image or heading). Good is under 2.5s. The quickest wins: properly sized images with explicit width/height, preloading hero images with <link rel='preload'>, and serving images in WebP/AVIF.
CLS: the silent revenue killer
Cumulative Layout Shift measures unexpected visual movement. A CLS score above 0.1 means elements are jumping around as the page loads. The business impact: users click the wrong element. They click 'close ad' when they meant to click 'add to cart'.
The fix is almost always the same: add explicit dimensions to images and embeds, reserve space for late-loading ads, and set font-display: optional or swap on custom fonts.
“Every millisecond of LCP above 2.5 seconds costs conversion rate. The correlation is consistent enough that Google publishes it.”
What to use for monitoring
- PageSpeed Insights — field data from Chrome UX Report, not just lab. Use this.
- Search Console Core Web Vitals report — shows which URLs are failing across your site, not just the homepage.
- Vercel Speed Insights — real-user data if you're on Vercel, updated continuously.
- Web Vitals Chrome extension — for diagnosing a specific page while logged in (bypasses cached states).
Check monthly. Set a calendar reminder. The third-party script that broke your INP in November wasn't there in June.