LCP, INP and CLS explained without jargon — what each one measures, what usually breaks it, and the fixes that make the biggest difference on a marketing site.
Core Web Vitals are Google's three measurements of how a page feels to use. They are a modest ranking factor and a significant conversion factor — a slow page loses visitors regardless of where it ranks. The good news is that on most marketing sites, a handful of fixes account for nearly all the improvement.
The three metrics
LCP — Largest Contentful Paint
How long until the biggest thing on screen appears. Usually your hero image or headline. Target: under 2.5 seconds.
Common causes of a bad LCP: an enormous unoptimised hero image, a web font blocking text rendering, or a slow server response before anything can start.
INP — Interaction to Next Paint
How quickly the page responds when someone taps or clicks. Target: under 200 milliseconds.
Almost always caused by too much JavaScript competing for the main thread — typically third-party tags rather than your own code.
CLS — Cumulative Layout Shift
How much the page jumps around while loading. Target: under 0.1.
Caused by images without dimensions, ads or embeds injected after load, and fonts that swap and change text size.
The fixes that actually move the numbers
Set width and height on every image
The single highest-value fix for CLS. With dimensions declared, the browser reserves the space before the image arrives and nothing jumps. It costs nothing.
Serve images at the size they are displayed
A 4,000-pixel-wide photograph rendered into a 600-pixel slot wastes most of the bytes. Modern formats and correctly sized variants routinely cut image weight by 70%.
Audit your third-party tags
Analytics, chat widgets, heatmaps, A/B tools, ad pixels. Each one is code you did not write running on your critical path. Load what you can after interaction, and remove anything nobody has looked at in six months.
Preconnect to font hosts, and use font-display
Text that cannot render until a font downloads is a self-inflicted LCP problem. Preconnect to the font origin and let text render in a fallback immediately.
Measure field data, not just lab data. A lighthouse score on your laptop over office wifi tells you very little. Google's Search Console reports what real visitors on real devices experienced — that is the number that counts.
Where diminishing returns start
Once all three metrics are in the green for the majority of real visits, further optimisation is usually not the best use of your budget. At that point the constraint on conversion is almost always the content, the offer or the form — not another 200ms.
A pragmatic order of work
- Check Search Console for which pages fail and on which metric
- Fix image dimensions and sizes sitewide — usually the biggest single win
- Cut or defer third-party scripts
- Sort out fonts
- Re-measure after four weeks of real traffic
If you are commissioning a new build, make these targets part of the acceptance criteria rather than something to fix later — see what a business website should cost for how that fits into scope.
Frequently asked questions
Do Core Web Vitals really affect rankings?
They are a real but modest signal, and they act as a tie-breaker between pages of similar relevance. Content quality and relevance matter far more. The stronger argument for fixing them is conversion, not ranking.
How often should I check them?
Monthly in Search Console, and after any significant release. Field data covers a rolling 28-day window, so a regression takes a few weeks to show fully.
My score is fine on desktop but poor on mobile. Why?
Mobile devices have less processing power and often slower connections. Google evaluates mobile separately, and it is the one that usually matters — test on a mid-range phone on a normal connection, not a flagship on wifi.
