Core Web Vitals explained without the jargon

What Core Web Vitals actually measure

Core Web Vitals are three measurements Google uses to describe how your website feels to a real person using it. Not how it looks, and not the words on the page, but the experience: how quickly the main content appears, how promptly the page responds when someone taps or clicks, and whether things stay put instead of jumping around while the page loads. Google gave these three things names and turned them into scores, which is where the jargon starts. Strip the labels away and each one answers a question a visitor would ask without thinking: is it loading, is it working, and why did the button move just as I went to press it.

Google publishes a good, a needs improvement and a poor band for each metric, and it judges you on real visits from real people rather than a single test on a fast office connection. That last point matters more than it sounds, and we come back to it further down.

The three metrics, one at a time

Here is each one in plain terms, with the score Google treats as good and the usual reason it slips.

Largest Contentful Paint (LCP): how fast the page loads

LCP times how long it takes for the biggest thing on screen to appear, usually your main image, a hero banner or a large block of text. It is the moment the page stops looking blank and starts looking like the page. Google treats anything at or under 2.5 seconds as good, and anything over 4 seconds as poor. Slow LCP is almost always about weight and hosting: an oversized photo straight off a phone camera, a slow server, or too much code loading before the important part gets a look in.

Interaction to Next Paint (INP): how fast it responds

INP measures the gap between someone interacting with your page, tapping a menu, opening a booking form, pressing add to basket, and the page visibly reacting. If nothing happens for a beat, that lag is what INP captures. Under 200 milliseconds is good, over 500 is poor. INP replaced an older metric called First Input Delay in 2024, and it is stricter, because it looks at interactions across the whole visit rather than just the first one. Poor scores here usually come from heavy scripts, often bolted-on plugins and tracking tools, tying up the page in the background.

Cumulative Layout Shift (CLS): how steady it stays

CLS is the annoying one everybody recognises. You go to tap a link, an image or advert loads in above it, everything shifts down, and you tap the wrong thing. CLS puts a number on how much the page moves around while it settles. A score of 0.1 or below is good, above 0.25 is poor. The usual causes are images and adverts without a reserved space, and web fonts that swap in late and push the text around.

Why this matters for a business in Cornwall

There are two honest reasons to care. The first is Google. Core Web Vitals are part of how Google decides which pages to show, and while they are not the single biggest factor, relevant content still comes first, they can be the tie-breaker between you and a competitor offering the same thing. If two Truro cafes, tradespeople or holiday lets are otherwise evenly matched, the faster, steadier site has the edge.

The second reason is the visitor, who does not know or care what LCP stands for. They just leave. Someone stood outside your shop on Boscawen Street checking your opening hours on their phone, or a family planning a trip searching for a cottage in the evening, will give a slow page a few seconds at most before going back to the results and tapping the next one.

Cornwall adds a wrinkle a distant agency might overlook. Mobile coverage across the county is uneven, and plenty of your visitors are browsing on patchy mobile data rather than fast home broadband, sometimes while travelling down for the summer. A page that is heavy but fine on the office wifi can be genuinely painful on a phone halfway along the A30. Building for the slowest realistic connection, not the fastest, is the whole game. It also means your busiest, most valuable traffic, the summer season when visitor numbers swell, often arrives in exactly the conditions your site handles worst.

It helps to remember what the score is standing in for. Nobody visits your website to admire its loading time. They come to find your number, book a table, check whether you cover their village, or buy something. Every extra second before the page is usable is another chance for them to give up before they reach the point of the visit. Core Web Vitals are simply a way of measuring the friction between arriving and getting what they came for.

Field data versus lab data, and why your scores wobble

This is the part that confuses most owners, so it is worth being plain. There are two kinds of Core Web Vitals score. Lab data is a single test run in controlled conditions: handy for diagnosis and instant to get. Field data is the real thing, gathered from actual Chrome users who visited your site over the previous 28 days, and it is the data Google actually uses to judge you. Google measures at the 75th percentile, which is a longer way of saying at least three quarters of your visits need a good score for the page to pass.

This explains two things owners often find baffling. First, your score can change even when you have not touched the site, because the mix of visitors and their devices changed. Second, a brand new or very quiet page may show no field data at all, simply because not enough people have visited it yet. Neither is a fault. It just means the numbers settle over weeks, not minutes.

How to check your own site

You can get a first read in a few minutes, for free, and with no technical knowledge. Two tools cover almost everyone. Run them on a phone as well as a computer, because the mobile score is usually the weaker one and the one most of your Cornwall visitors will experience. If a page sits comfortably in the green on all three metrics, you are in good shape and your effort is better spent elsewhere. If it is amber or red, these tools will point you at the cause.

• PageSpeed Insights: paste your web address into Google's free PageSpeed Insights tool and it reports both the lab and field scores for that page, colour coded, with a plain list of what is holding it back. Check your home page and one or two of your busiest pages, and use the mobile tab, not just desktop.
• Google Search Console: if your site is set up in Search Console, and it should be, the Core Web Vitals report there groups every page into good, needs improvement and poor using real visitor data. It is the closest thing to Google's own view of your site.

What actually slows a site down

Nearly every poor score traces back to a short list of culprits. None of them are mysterious once you know to look.

• Huge images: a photo straight off a modern phone can be several megabytes, far more than a web page needs. Resizing and compressing images, and serving them in a modern format, is the single biggest win for most small sites.
• Cheap or overloaded hosting: if the server is slow to respond, every visitor waits before anything can even begin to load. This is often the hidden cause behind a poor LCP.
• Too many plugins and scripts: every add-on, chat widget, popup, tracker and social feed adds weight and work. Sites that have collected features over the years are usually carrying more than they use.
• No caching: caching lets the server and returning visitors reuse work instead of rebuilding the page from scratch every time. A site without it is doing needless heavy lifting on every visit.
• Layout that jumps: images, adverts and embeds with no reserved size, and fonts that load late, all shove the page around and wreck your CLS.

What you can do, and when to get help

You will notice none of that involved changing what your site says or how it looks to a careful eye. That is the encouraging part. Improving Core Web Vitals is mostly plumbing: the visible page can stay exactly as you designed it while it gets lighter and steadier underneath.

Some of this is genuinely within reach of an owner who is comfortable in their own website. If you run WordPress or a similar system, you can resize and compress images before uploading them, remove plugins and widgets you no longer use, and turn on a reputable caching option. Those three alone move a lot of sites from amber to green.

The rest tends to need someone who can get under the bonnet: tuning or moving hosting, reworking how fonts and scripts load, reserving space so the page stops shifting, and reading the diagnostics so you fix the real cause rather than guessing. There is little point pouring hours into shaving milliseconds off a page that already passes, and equally little point ignoring a home page stuck in the red while you wonder why enquiries are thin. The skill is knowing which is which.

That is the work we do as a Truro studio: measuring what real visitors actually experience, finding the handful of things holding a site back, and putting them right without disturbing the design you already have. If your site feels sluggish on a phone, or your scores are stubbornly amber, it is usually a few clear problems rather than a hundred small ones.

One last thing worth saying, because it saves a lot of wasted effort: you are not chasing a perfect score. Some tools show a headline number out of 100, and it is tempting to treat 100 as the target. It is not. The goal is a good rating on all three Core Web Vitals for real visitors, on mobile in particular. A page that comfortably passes is doing its job, and the hours you might spend squeezing out the last few points are almost always better spent on your content, your photos or your enquiry form.