Site speed test

Site speed test

A guide and Core Web Vitals checklist (LCP, CLS, INP) and what affects site speed.

Tools · Performance

Site speed is measured through Core Web Vitals, the three metrics Google uses: LCP (how fast the main element loads), CLS (how stable the layout stays while loading) and INP (how fast the site responds to interactions). This guide shows what affects each metric and what to check for a fast site.

  • LCP (Largest Contentful Paint): how long the main visual element takes to load. Target: under 2.5 seconds.
  • CLS (Cumulative Layout Shift): how stable the layout stays while the page loads. Target: under 0.1.
  • INP (Interaction to Next Paint): how quickly the page responds to user interactions. Target: under 200 ms.
  • Affected by: image weight, fonts, third-party scripts and the space reserved for images and other elements.

How does it work?

Measure your starting point with a tool such as PageSpeed Insights or Lighthouse. They report LCP, CLS and INP for the page you test.

For a good LCP: optimize and compress images, load the main (hero) image with priority, and avoid scripts that block rendering.

For a good CLS: reserve fixed dimensions for images and ad slots, and load fonts with font-display: swap so text stays visible while the font loads.

For a good INP: reduce the amount of JavaScript that runs when users interact with the page, and defer non-essential scripts until after the initial load.

Frequently asked questions

What are Core Web Vitals?

Core Web Vitals are three user-experience metrics defined by Google: LCP (how fast the main content loads), CLS (visual stability) and INP (how fast the page responds to interactions). They are among the page experience signals Google uses in ranking.

How fast should a website load?

The recommended targets are LCP under 2.5 seconds, CLS under 0.1 and INP under 200 milliseconds. Google considers a page to have a good experience when it meets these values for most of its visits.

What slows a website down most often?

Common causes are large, uncompressed images, too many third-party scripts (chat widgets, analytics, advertising), render-blocking fonts, and missing size settings for images, which make the layout jump.

How can I check my site's speed?

Use a free tool such as Google PageSpeed Insights, or Lighthouse, which is built into Chrome DevTools. Enter the page address and you get Core Web Vitals results along with suggested fixes.

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