Core Web Vitals are the three metrics Google uses to judge whether a page feels fast and stable to real visitors. They are measured from real Chrome users, reported in Search Console, and used as a ranking signal. If you only track three performance numbers, track these.
Largest Contentful Paint (LCP)
LCP measures how long it takes for the largest visible element, usually a hero image or headline, to render. It is the closest single number to the question a visitor asks: has this page loaded yet?
- Good: 2.5 seconds or less. Poor: over 4 seconds.
- Most common causes: slow server response, render-blocking CSS and JavaScript, and oversized hero images.
- Biggest wins: preload the hero image, serve modern formats like AVIF or WebP, and defer non-critical scripts.
Interaction to Next Paint (INP)
INP replaced First Input Delay in 2024. It measures how quickly the page visibly responds after a click, tap, or key press, across the whole visit rather than just the first interaction.
- Good: 200 milliseconds or less. Poor: over 500 milliseconds.
- Most common causes: long JavaScript tasks, heavy third-party tags, and large DOM updates.
- Biggest wins: break up long tasks, load analytics and chat widgets after the page is interactive, and remove unused scripts.
Cumulative Layout Shift (CLS)
CLS measures how much content jumps around while the page loads. Every time a button moves just as someone goes to tap it, CLS goes up and trust goes down.
- Good: 0.1 or less. Poor: over 0.25.
- Most common causes: images without dimensions, late-loading ads and embeds, and web fonts swapping in.
- Biggest wins: reserve space for every image and embed, and use font-display with size-adjusted fallbacks.
Lab data versus field data
How to measure your own site
- Open PageSpeed Insights and enter your URL to see both field and lab data.
- Check the Core Web Vitals report in Google Search Console to find groups of failing URLs.
- Use Chrome DevTools Performance panel to find the exact element or script causing a problem.
LightLoader automates the most common fixes on all three metrics, including image prioritization, script deferral, and layout reservation, from a single script tag. Whatever tool you use, measure before and after so you know what actually moved.