What are Core Web Vitals?
You will find "core web vitals explained" in a hundred technical articles, but the idea itself is simple. Core Web Vitals are three numbers Google records from real people visiting your website, covering the three things visitors feel most: how long the main content takes to appear, how quickly the page reacts when they tap or click, and how much the layout shifts about while loading. Google publishes exact pass marks for each at web.dev, and uses the results as one of its ranking signals.
Think of them as an MOT for your website. Nobody buys a car because it passed its MOT, but a failed one keeps the car off the road. The same logic applies here: passing does not make a website good, but failing quietly costs you visitors before your content gets a hearing.
LCP: how long the main course takes to arrive
Largest Contentful Paint measures how long it takes for the biggest visible element on the screen, usually your main image or headline, to actually appear. Not the spinner, not the background colour: the thing the visitor came for.
The restaurant analogy holds up well. LCP is the time between placing your order and the main course landing on the table. Bread rolls and a filled water glass are nice, but you have not been fed until the plate arrives. A page can technically be "loading" while showing nothing that matters; LCP ignores the bread rolls and times the plate. Google's guidance says the plate should arrive within 2.5 seconds. In practice, slow LCP usually means oversized images or a server that is slow to respond in the first place.
INP: does the page respond when you press the button?
Interaction to Next Paint measures how quickly the page visibly reacts when a visitor taps, clicks, or types. It replaced the older First Input Delay metric in March 2024 and is deliberately stricter: it considers interactions across the whole visit, not just the first one.
Everyone has stood at a pedestrian crossing and pressed the button, then wondered if it registered, then pressed it four more times. That uncertainty is exactly what INP measures. When a visitor taps your menu and nothing visibly happens for half a second, they do not think "the JavaScript is busy". They think the site is broken, and they tap again, or leave. Google's threshold is a visible response within 200 milliseconds, which is roughly the boundary where a delay stops feeling instant. Slow INP is nearly always caused by too much script work competing for the device's attention.
CLS: does the page hold still while you use it?
Cumulative Layout Shift measures how much the page moves around while loading. It is a score rather than a time: 0 means nothing budged, and higher numbers mean more lurching.
Imagine reading a newspaper where the article slides down the page each time you reach the second paragraph, because adverts keep being pasted in above it. That is layout shift, and its most expensive form is the moving button: you go to tap "Read reviews", the layout jumps as a late-loading banner barges in, and you tap "Buy now" instead. Google counts a score below 0.1 as good. The usual culprits are images without reserved space, late-arriving adverts and embeds, and fonts that swap size after loading. It is also the failure visitors interpret most personally, because it punishes them mid-action.
The thresholds: good, needs improvement, and poor
Google publishes precise boundaries for all three metrics in its Web Vitals documentation. To pass, a page must hit the "good" band for at least 75 per cent of real visits, so occasional slow loads on bad connections will not fail you.
| Metric | What it measures | Good | Needs improvement | Poor |
|---|---|---|---|---|
| LCP (Largest Contentful Paint) | Time for the main content to appear | 2.5 seconds or less | 2.5 to 4 seconds | Over 4 seconds |
| INP (Interaction to Next Paint) | Delay before the page visibly responds | 200 milliseconds or less | 200 to 500 milliseconds | Over 500 milliseconds |
| CLS (Cumulative Layout Shift) | How much the layout moves while loading | 0.1 or less | 0.1 to 0.25 | Over 0.25 |
Treat the "needs improvement" band as exactly what it says: not a crisis, but not a pass. Pages sitting there tend to drift into "poor" as more content, plugins, and pixels accumulate, because website weight only ever moves in one direction unless someone is watching it.
Field data versus lab data: the review and the mystery shopper
Every Core Web Vitals report you run shows two kinds of evidence, and knowing the difference saves a lot of confusion. Field data is gathered from real Chrome users visiting your site over the previous 28 days: real phones, real connections, real impatience. Lab data is a single simulated test on a standardised mid-range device, run at the moment you ask.
Field data is the customer reviews; lab data is the mystery shopper. The reviews tell you the verdict, because they reflect genuine experience at scale, and they are what Google's rankings use. The mystery shopper tells you the detail, because the lab test comes with an itemised list of what went wrong and where. Two consequences follow. A shiny lab score means nothing if the field data fails, and small sites often have no field data at all, simply because there are not yet enough visitors to sample. Neither is cause for panic; it just tells you which report to read.
How to check your website's Core Web Vitals free
You need no tools budget and no technical setup. Go to PageSpeed Insights, paste in your address, and read the "Discover what your real users are experiencing" section at the top: that is your field data, with a plain pass or fail against each metric. Check the mobile tab first, because that is where most visits and most failures happen. Test your homepage plus your two or three most important pages, since results vary page by page.
If you use Google Search Console, its Core Web Vitals report shows the same field data grouped across your whole site, which is the easiest way to watch trends over time. And if you want the metrics checked alongside everything else that decides whether a website earns enquiries, our CRISP audit includes speed as one of its five scored dimensions.
What to do about failures without becoming a developer
Your job as an owner is diagnosis and direction, not implementation. Each failing metric points at a known family of causes, so you can brief the right fix instead of buying speed work blind.
Failing LCP? The usual suspects are oversized images, slow hosting, and no caching, and the first of those is frequently fixable through your platform's own settings. Our breakdown of why websites are slow walks through all ten common causes and flags which fixes are an afternoon versus structural. Failing INP? That is script weight: too many plugins, widgets, and trackers, and the cheapest first step is removing the ones nobody can defend. Failing CLS? Ask whoever built the site to reserve space for images and embeds, and to sort the font loading; both are routine, bounded jobs.
Where owners get burned is buying "optimisation" without a diagnosis, or optimising a site whose theme is the actual problem. A proper website audit tells you which metric fails, why, and whether the fix is a tweak or a rebuild decision, before any money moves. If you just want the headline verdict right now, run the free audit and you will have a scored answer in about a minute.
Frequently asked questions
What happened to First Input Delay (FID)?
Google retired First Input Delay in March 2024 and replaced it with Interaction to Next Paint. FID only measured the delay before the very first interaction started being processed, which let sluggish pages score well. INP is stricter: it looks at the responsiveness of interactions across the whole visit, so it reflects how the page actually feels to use, not just its first impression.
Do Core Web Vitals affect Google rankings?
Yes, Google confirms Core Web Vitals feed into its ranking systems, but the effect is modest. Google's spokespeople have consistently described it as closer to a tiebreaker between similar pages than a major factor, and relevance still wins. The stronger reasons to care are human: visitors abandon slow, jumpy pages, and that costs enquiries regardless of where you rank.
Why does PageSpeed Insights show no field data for my site?
Field data comes from real Chrome users, and Google only publishes it when a page or site receives enough traffic to produce a reliable sample. Many small business sites fall below that bar, so the report shows lab results only. That is not a penalty or a problem in itself; it just means you should lean on the lab diagnostics and your own testing instead.
Can I pass Core Web Vitals on WordPress, Wix, or Squarespace?
Yes, plenty of sites on mainstream platforms pass. The platform matters less than what has been loaded onto it: lean themes, right-sized images, and restraint with plugins and embeds go a long way on any system. That said, some heavy themes and page builders make passing genuinely difficult, and in those cases the honest fix is structural rather than another optimisation plugin.