If you have ever run your website through Google’s PageSpeed Insights, you will have seen three acronyms near the top of the report: LCP, INP and CLS. Together they make up Google’s Core Web Vitals, a set of measurements that describe how a page feels to a real visitor. They are not the only thing that matters for rankings, but they are one of the few technical signals Google has documented openly, and they line up closely with whether people stay on your site or give up.
This guide explains each metric without the jargon, what Google counts as a “good” score, and where small business sites usually lose points.
Largest Contentful Paint (LCP): how quickly the main content appears
LCP measures the time it takes for the biggest visible element on the screen, usually a hero image or a large heading, to finish loading. Google’s threshold for “good” is 2.5 seconds or less.
The most common causes of a slow LCP are:
- Oversized hero images. A 4,000-pixel photo straight from a camera or stock site, displayed in a 1,400-pixel space.
- Slow hosting. If the server takes a second to respond before anything else can start, you have already used up a large part of your budget.
- Render-blocking files. Fonts, sliders and scripts that must load before the browser is allowed to draw the page.
- Lazy-loading the wrong image. Lazy-loading is great for images further down the page, but applying it to the hero image delays the very thing LCP is timing.
Interaction to Next Paint (INP): how responsive the page feels
INP replaced the older First Input Delay metric in 2024. It looks at how long the page takes to react visibly when someone taps a button, opens a menu or types into a form. A “good” INP is 200 milliseconds or less.
Poor INP is almost always caused by too much JavaScript running at once. Page builders with dozens of add-ons, chat widgets, multiple tracking tags and heavy animation libraries all compete for the same processing time, and on a mid-range phone the delay becomes very noticeable.
Photo by William Warby on Unsplash
Cumulative Layout Shift (CLS): whether things jump around
CLS measures unexpected movement. You have probably experienced it: you go to tap a link and, at the last moment, an image or cookie banner loads and pushes everything down, so you tap something else. Google wants a CLS score of 0.1 or lower.
The usual culprits are images and embeds without width and height set, web fonts that swap in at a different size, and banners injected at the top of the page after it has loaded. Each of these has a straightforward fix once it has been identified.
Lab data versus field data
PageSpeed Insights shows two kinds of results, and it is worth knowing the difference:
- Field data comes from real Chrome users who visited your site over the previous 28 days. This is what Google uses when it assesses Core Web Vitals. Smaller sites may not have enough visitors for this section to appear.
- Lab data is a simulated test run at that moment on a throttled connection. It is the score out of 100 most people quote, and it is the best tool for diagnosing problems and checking fixes.
When we rebuilt our own website we treated performance as a design requirement rather than an afterthought, and it now scores 100 for performance in the PageSpeed Insights lab test. That did not come from a plugin; it came from compressed images served in modern formats, self-hosted fonts, minimal JavaScript and good hosting.
Where to start if your scores are poor
- Run your homepage and your most important service page through PageSpeed Insights on the mobile setting.
- Look at which of the three metrics is failing and read the “Diagnose performance issues” list beneath it.
- Fix images first. Resizing and converting them to WebP or AVIF is often the single biggest improvement.
- Remove plugins, widgets and tracking scripts you no longer use.
- If the server response time is slow, speak to your host or consider moving.
If your site is built on an older platform that cannot be made fast without fighting it, it may be time to look at a rebuild. Our legacy system modernisation work often starts with exactly this kind of performance audit.
Want a second opinion on your scores? Get in touch and we will tell you honestly what is holding your site back.
