Core Web Vitals in plain English: what actually makes a site fast
“Your site needs to be faster” is easy to say and hard to act on. Google has at least made it measurable: three numbers, called Core Web Vitals, that describe how a page feels to the person waiting for it. They feed into search rankings, but more importantly they correlate with whether someone stays.
The three numbers
LCP — how long until the main thing appears
Largest Contentful Paint measures when the biggest element in view finishes rendering: usually your hero image or headline. It answers “when does this page look like something?” Aim for under 2.5 seconds.
The usual culprit is an enormous hero image, followed by fonts that block rendering and slow server responses.
INP — how quickly the page answers you
Interaction to Next Paint measures the lag between a tap or click and the page visibly responding. It replaced the older First Input Delay because it looks at every interaction, not just the first. Aim for under 200 milliseconds.
This one is almost always JavaScript. Too much of it, running at the wrong moment, blocks the browser from responding while it works.
CLS — how much the page moves under your thumb
Cumulative Layout Shift measures content jumping around as things load. If you have ever gone to tap a link and hit an advert that appeared a moment earlier, you have felt bad CLS. Aim for under 0.1.
The fix is nearly always reserving space: give images explicit width and height, and don't inject banners above content that has already rendered.
What actually moves the needle
- Compress and size your images properly. This is the single biggest win on most small business sites. A 4MB photo straight off a phone, scaled down in the browser, still downloads all 4MB. Modern formats like WebP typically cut that by 70% or more with no visible difference.
- Ship less JavaScript. Every plugin, chat widget, tracking pixel and carousel adds code that must download, parse and run. Audit what is actually earning its place.
- Load fonts sensibly. Use
font-display: swapso text is readable in a fallback font while the real one arrives, and limit yourself to the weights you genuinely use. - Reserve space for anything that loads late. Images, embeds, adverts, cookie banners.
- Host somewhere fast, close to your customers. A CDN puts your files near the visitor instead of on one server on the other side of the world.
How to measure it honestly
Run PageSpeed Insights on your own URL. Pay most attention to the field data section if it appears: that is real Chrome users on real connections, not a simulation. Lab scores are useful for diagnosing, but field data is the truth.
Test on a mid-range phone over mobile data, not just your laptop on office broadband. That is closer to how most people will actually meet your site.
A note on chasing 100
A perfect score is not the goal. Going from 45 to 85 changes how the site feels; going from 92 to 97 usually does not, and the effort is often better spent on the page itself. Fix the big things, then get back to your business.