Key takeaways
- Core Web Vitals are three measures of real user experience: loading (LCP), responsiveness (INP) and visual stability (CLS).
- Images, third-party scripts and heavy plugins are the most common causes of a slow site.
- Measure with real-user data from Search Console and PageSpeed Insights before changing anything.
What Core Web Vitals measure
Google uses three measures of how a page feels to real visitors. A page is judged on the 75th percentile of visits, so a few slow loads do not sink it, but a pattern does.
| Metric | What it measures | Good | Poor |
|---|---|---|---|
| Largest Contentful Paint (LCP) | How long the main content takes to appear | 2.5 seconds or less | Over 4 seconds |
| Interaction to Next Paint (INP) | How quickly the page responds to taps and clicks | 200 milliseconds or less | Over 500 milliseconds |
| Cumulative Layout Shift (CLS) | How much content jumps as the page loads | 0.1 or less | Over 0.25 |
How to measure first
- Open the Core Web Vitals report in Google Search Console. It shows real visitor data, grouped by pages with similar problems.
- Run your key pages through PageSpeed Insights. Look at the real-user section first, and the lab results second. The lab test helps you find causes.
- Test on a phone over a mobile connection, because that is where most of your visitors are.
- Write the numbers down so you can see what each change did.
Fix 1: Compress and resize images
Large images are the most common reason a page loads slowly. Resize each image to the size it is shown at, compress it, and use modern formats such as WebP or AVIF. Load images below the fold lazily, but never the main image at the top, which would slow LCP.
Fix 2: Reserve space for everything that loads late
Give images, videos, adverts and embeds a fixed width and height so the layout does not jump when they arrive. This is the main cure for a high CLS score and it also makes the page feel steadier.
Fix 3: Trim plugins and third-party scripts
Every plugin, chat widget, tracker and font library adds code to download and run. List them all, remove what you do not use, and load the rest only on the pages that need them. This is also the main way to improve INP, since less code competes for the browser’s attention.
Fix 4: Improve hosting and caching
If the server takes a long time to respond, nothing else can make the page fast. Use hosting that suits your traffic, switch on page caching and put a content delivery network (CDN) in front of images and files.
Fix 5: Handle fonts carefully
Use as few font files as you can, load only the weights you use, serve them from your own site in WOFF2 format and set font-display to swap, so text shows at once. Preload the one or two files used above the fold.
Fix 6: Defer what is not needed straight away
Scripts and styles that block the first display slow LCP. Load non-essential JavaScript after the page, split large bundles and avoid running long tasks that freeze the page after a tap.
What order to work in
- Start with images and the top-of-page image in particular.
- Remove unused plugins and scripts.
- Fix layout shifts by adding dimensions.
- Check hosting and caching.
- Tidy fonts and defer scripts last, and test again after each change.
Speed is one part of a wider search plan. See how it fits in What is AI SEO? and our web development service.