Let me save you a long debate with a developer. Core Web Vitals will not, on their own, rocket you up the rankings. Google has been consistent that page experience is one of many signals and that it will still show genuinely relevant content even when the experience is sub-par. John Mueller has described Core Web Vitals as more than a tie-breaker, but not a replacement for relevance. So the honest framing is this: a small, conditional ranking nudge, and a real, measurable conversion lever. Optimise them for the second reason and the first comes along for free.

The three metrics, and the numbers that matter

There are three Core Web Vitals, each with a "good" threshold measured at the 75th percentile of your real users:

  • LCP (Largest Contentful Paint), loading: good is 2.5 seconds or less.
  • INP (Interaction to Next Paint), responsiveness: good is 200 milliseconds or less. INP replaced FID in March 2024, so if your reports still mention First Input Delay, they are out of date.
  • CLS (Cumulative Layout Shift), visual stability: good is 0.1 or less.

A URL passes when 75% of real visits hit "good" on all three, in Google's field dataset (CrUX).

Field data is the truth, lab data is the rehearsal

This trips up a lot of teams. Field data comes from real Chrome users over a rolling 28-day window, reported at the 75th percentile, and it is what ranking actually uses. Lab data, from Lighthouse, is a single simulated load, brilliant for debugging and not the source of truth. You can post a Lighthouse score of 95 and still fail LCP in the field, because the lab does not feel a real mid-range phone on a patchy connection. Trust the field. Use the lab to find out why.

The fixes that actually move each metric

LCP: the usual culprit is the hero image. Compress it, serve modern responsive formats, add fetchpriority="high" and preload the element, cut your server response time with caching, and put assets behind a CDN.

INP: this is a JavaScript problem nine times out of ten. Break up long tasks, defer or remove third-party scripts, avoid heavy synchronous event handlers, and yield to the main thread so the page can respond to the tap it just received.

CLS: reserve space. Set explicit width and height (or an aspect-ratio) on images, ads and embeds, hold the slot for anything that loads late, and handle web fonts so text does not jump as they swap in.

Two myths to bin

  • "A perfect Lighthouse score boosts rankings." Lighthouse is lab data. Ranking reads field data. They are not the same number.
  • "Fix Core Web Vitals and we will rank." No. Relevance and links still do the heavy lifting. Vitals help most when you and a competitor are otherwise neck and neck.
Sell speed as a conversion win, not a ranking trick. Faster LCP and INP reduce bounce and lift conversions, and they happen to keep Google content too.

A Technical SEO Audit measures your vitals on real-user field data, separates the lab noise from the field reality, and hands your developers a prioritised, specific fix list rather than a vague "improve performance." That is usually the difference between a report that gets read and one that gets shipped.