When we review a slow website for a client, we report against three measurements that Google calls Core Web Vitals. They are taken from real visitors using Chrome, and Google uses them as one signal in search ranking. More to the point, they describe whether the page feels quick or frustrating.
The three measurements
- Largest Contentful Paint is how long the main content takes to appear. Good is 2.5 seconds or less.
- Interaction to Next Paint is how quickly the page responds when someone taps or clicks. Good is 200 milliseconds or less.
- Cumulative Layout Shift is how much the page jumps around while it loads. Good is a score of 0.1 or less.
A page passes when at least three quarters of visits meet all three.
Lab scores and real visitors
A testing tool gives you a lab score from one simulated visit. Core Web Vitals come from your actual visitors, on their phones and their connections. A site can score well in the lab and still fail in the field. Look at both, and trust the field data.
What usually causes a fail
- Very large images, especially the main image at the top of the page.
- Too many third-party scripts: chat widgets, trackers, pop-ups and embedded video.
- Fonts and banners that load late and push the content down.
- Slow hosting, or pages that are built from scratch for every visitor instead of being cached.
- Page-builder plugins that load far more code than the page uses.
Where to start
Measure your most visited pages on a phone. Fix the main image and remove scripts nobody is using. Those two changes account for most of the improvement we see. Then decide whether the hosting or the theme needs attention.
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