What Core Web Vitals measure
Core Web Vitals are Google’s three numbers for how a page feels to a real user: how fast the main content appears, how quickly the page reacts to a tap, and whether things jump around while it loads. They are part of Google’s ranking signals, but the honest reason to care is simpler: a slow, jumpy page loses visitors before they read a word. The three metrics, in plain terms:
| Metric | What it measures | Good |
|---|---|---|
| LCP (Largest Contentful Paint) | How fast the main content appears | under 2.5 s |
| INP (Interaction to Next Paint) | How quickly the page reacts to a tap or click | under 200 ms |
| CLS (Cumulative Layout Shift) | How much the layout jumps while loading | under 0.1 |
The official thresholds and details are on Google’s web.dev. Note that INP replaced the older FID metric in 2024, so if you read an old guide mentioning FID, INP is its stricter successor.
What usually drags each one down
- LCP (slow main content): usually a big unoptimised hero image or a slow server. Compress the image, serve modern formats (WebP), and make sure the main image is not lazy-loaded.
- INP (sluggish response): usually too much JavaScript, often from plugins or trackers running on every tap. Remove what you do not need.
- CLS (jumpy layout): usually images or ads without reserved space, or web fonts swapping in late. Set width and height on images and reserve space for anything that loads in.
How to measure it honestly
Run your page through PageSpeed Insights or the Lighthouse tab in Chrome. Two things to know: the lab score is a single simulated run and jumps around a little, so treat it as a guide. The field data, if Google has enough traffic for your site, is what real visitors actually experienced, and that is the one that counts for ranking. Small local sites often have no field data yet, in which case the lab score is your best signal.
What is realistic for a small site
You do not need a perfect 100. Aim to get all three into the green: a hero image under a few hundred kilobytes, no unused plugins, and images with fixed dimensions will get most local sites there. The single biggest win is almost always the main image. If your site feels slow on a phone, our guide on a slow mobile website walks through the usual culprits.
Why it is worth doing
Speed is one of the four things Google and AI systems reward, alongside clear content, structure and trust. A fast page is easier to crawl, keeps visitors long enough to convert, and quietly supports everything else you do for visibility. It is groundwork, not a trick, and it rarely needs a redesign.
Frequently asked questions
Do I need a score of 100?
No. Getting all three metrics into Google’s green range is the goal, not a perfect score. For most local sites, fixing the main image and removing unused plugins is enough.
What happened to FID?
Google replaced First Input Delay with INP in 2024. INP is a stricter, more complete measure of how responsive the page feels. If a guide still mentions FID, it is out of date.
My lab score changes every time I test. Why?
The lab score is a single simulated run and varies a little between tests. Use it as a guide. Field data from real visitors, where available, is the stable, ranking-relevant number.
Is this really worth it for a small local business?
Yes, because a slow page loses visitors before they read anything. And the fixes are usually small: compress the hero image, drop unused plugins, set image dimensions.