Core Web Vitals for Developers Who Do Not Have Time
Three metrics decide how Google sees your site's speed. Here is what each one measures, what actually causes a bad score, and the fixes that give the biggest return.
Core Web Vitals are three numbers Google uses to describe how a page feels to a real user. They are not a vanity metric — they feed into ranking, and more importantly they correlate with people actually staying on your site.
LCP — Largest Contentful Paint
How long until the biggest visible element renders. Target: under 2.5 seconds.
Almost always it is one of three things:
- A hero image that is 3MB and unsized.
- A render-blocking stylesheet or font.
- A slow server response before anything can start.
The fixes, in order of return: compress and serve the hero image as WebP, add explicit width and height, preload it, and make sure nothing above it blocks rendering.
<link rel="preload" as="image" href="/images/hero.webp">INP — Interaction to Next Paint
How long the page takes to visibly respond after a tap or click. Target: under 200ms.
Bad INP is almost always heavy JavaScript running on the main thread. Every third-party script — chat widget, analytics, heat mapping — competes for the same thread as your click handler.
Load anything non-essential with defer, and question every widget the client asks for.
CLS — Cumulative Layout Shift
How much content jumps around while loading. Target: under 0.1.
The causes are short and predictable:
- Images without dimensions — the browser cannot reserve space.
- Web fonts swapping and changing text height. Use
font-display: swapwith a well-matched fallback stack. - Banners and ads injected above existing content.
Measure Real Users, Not Just Lighthouse
Lighthouse runs on a simulated device on your fast connection. Google ranks on field data from real Chrome users. Check Search Console's Core Web Vitals report — that is the number that counts.
The 80/20
Compress your images, set their dimensions, defer non-critical JavaScript, and enable server caching. On most sites those four things move all three metrics into the green.