Core Web Vitals: what still matters and what changed

Core Web Vitals are three measurements of how a page feels to use. They influence search rankings, though less than most articles imply, and they correlate strongly with revenue, which matters more.

The three, in plain terms

INP is the one that changed

It replaced First Input Delay, and it is a much harder test. FID measured only the delay before the browser began handling your first interaction. INP measures the full round trip — delay, processing, and the visual update — across every interaction on the page, and reports near the worst.

Sites that passed comfortably on FID often fail INP, and the usual cause is JavaScript doing too much work on the main thread when someone taps. Heavy frameworks, oversized event handlers and third-party scripts all show up here where they previously hid.

Why your lab score disagrees with reality

Lighthouse simulates a device and a connection. Field data is collected from real visitors on real hardware. They routinely disagree, and search engines use the field data.

So a perfect Lighthouse score means very little on its own, and a poor one on a site with good field data is not worth chasing. Look at the field section of PageSpeed Insights, or the Core Web Vitals report in Search Console.

What actually moves each one

• LCP: the hero image, almost always. Right format, right size, not lazy-loaded, and served from a CDN.
• INP: reduce main-thread JavaScript. Audit third-party scripts first — they are usually the largest contributor and the easiest to remove.
• CLS: set dimensions on images and reserve space for anything injected later. Banners and consent notices are the usual culprits.

A realistic target

Pass all three on mobile field data. That is achievable on most sites without a rewrite, and it is where the commercial benefit sits. Chasing a lab score of 100 past that point is optimising a number rather than an experience.