Google evaluates every page on three user-experience metrics called Core Web Vitals: how fast the main content appears, how quickly the page reacts when someone taps or types, and whether things jump around while loading. They influence rankings, and more importantly they influence whether visitors stay.
You do not need to become an engineer to manage this. You need to know what the three numbers mean, where to read them, and which fixes are worth paying for. That is this guide.
LCP — Largest Contentful Paint
LCP measures how long it takes for the largest visible element — usually a hero image or headline — to appear. Good is 2.5 seconds or less on mobile. It is the metric visitors feel most directly: a slow LCP is 'this site is slow'.
Typical causes: oversized hero images, slow hosting, render-blocking scripts and fonts loaded before content. Typical fixes: compress and resize images, serve modern formats, preload the hero image, use a CDN, defer non-critical scripts.
INP — Interaction to Next Paint
INP replaced First Input Delay in March 2024. It measures how quickly the page responds to interactions across the whole visit — tapping a menu, opening a filter, typing in a form. Good is 200 milliseconds or less.
INP suffers when the browser is busy running JavaScript: heavy trackers, chat widgets, sliders and page builders that ship far more code than the page needs. Fixes are about doing less on the main thread — removing unused scripts, loading widgets on demand and simplifying interactions.
CLS — Cumulative Layout Shift
CLS measures visual stability: content jumping as images, ads or banners load and push everything down. Good is 0.1 or less. It is the reason people tap the wrong button on slow connections.
Fixes: give every image and embed explicit dimensions, reserve space for banners and cookie bars, load web fonts without a flash of unstyled text, and avoid inserting content above what the user is already reading.
Where to read your numbers
Search Console's Core Web Vitals report shows which pages fail based on real visitors (field data). PageSpeed Insights shows both field data and a lab test for a single URL with specific recommendations. Field data is what Google uses; lab tests help you diagnose.
Check mobile first — it is where most visitors are and where scores are lowest. A site that is green on desktop and red on mobile is red.
What is worth fixing, in order
Start with images, since they usually cause LCP and CLS problems at once. Then remove or defer scripts you do not need — every third-party widget is a tax on INP. Then hosting and caching. Only then fine-tune fonts and CSS.
If the platform itself is the problem — a heavy theme, a page builder or an ageing CMS — optimisation has a ceiling. That is the point where a rebuild on a lean foundation is cheaper than another round of patches; our redesign checklist covers how to do that without losing rankings.
Key takeaways
- LCP ≤ 2.5s, INP ≤ 200ms, CLS ≤ 0.1 on mobile is the target; field data in Search Console is the judge.
- Images fix two metrics at once; third-party scripts are the usual INP culprit.
- Reserve space for everything that loads late to eliminate layout shift.
- Heavy platforms have an optimisation ceiling — sometimes the fix is a leaner rebuild.
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