Core Web Vitals: how to check your site speed

What Core Web Vitals are and their thresholds

They are three metrics Google collects from real Chrome users. LCP (Largest Contentful Paint) shows when the largest element above the fold appears. INP (Interaction to Next Paint) shows how fast the page responds to a tap, and CLS (Cumulative Layout Shift) whether content jumps around. The "good" thresholds:

• LCP under 2.5 s, poor above 4 s
• INP under 200 ms, poor above 500 ms
• CLS under 0.1, poor above 0.25

Google does not use the average but 75% of visits: if three out of four visitors land in "good", the metric is green. Mobile and desktop are assessed separately, and a group of pages takes the status of its worst metric. INP replaced FID on 12 March 2024, so a tool that still shows FID is out of date.

How to check Core Web Vitals in 5 minutes

Open PageSpeed Insights and choose the mobile view. The top part shows real visitors over the last 28 days from CrUX (the Chrome User Experience Report), and that is where the Core Web Vitals assessment lives. The bottom part is a Lighthouse test that simulates an average phone on a mobile network. Check not only the home page but also your most visited product or service page.

Search Console has a Core Web Vitals report for the whole site. It groups similar pages, shows mobile and desktop separately and uses the same 28-day data. After a fix, click "Validate fix": monitoring takes 28 days, so you will not see the result the next day.

Why the PageSpeed score and Core Web Vitals disagree

The 0-100 score is a single lab run: one device, one network and an empty browser cache. Real visitors have different phones, connections and often cached files. That is why you can see a green score with a failed assessment, and the other way round.

INP cannot be measured in the lab because it depends on when a person taps, so Lighthouse only shows TBT (Total Blocking Time) as a hint. The score changes every run due to server and network variance. Google Search cares about field data, while the lab is diagnostics: it shows what to fix.

When Google has no data about your site

CrUX only collects public, sufficiently visited pages from Chrome on desktop and Android, when the user allows usage statistics. Chrome on iPhone and Safari are not included. Without page-level data PageSpeed Insights shows origin data, and without that it shows nothing. For many small business sites this is the normal state, not an error.

In that case use the lab test as diagnostics and watch LCP and TBT on mobile. If you need real numbers, you can collect them yourself with Google's web-vitals library and send them to GA4. How Google assesses pages without CrUX data is not publicly documented.

Do Core Web Vitals affect Google rankings

Google states that its ranking systems use Core Web Vitals and recommends keeping them good. But good metrics do not guarantee the top spot, and the most relevant content is shown even with a weaker page experience. Assessment is mostly page by page.

In practice, between two equally relevant pages the faster one has an edge, but speed will not replace content. The bigger effect is not on rankings but on what happens once a visitor arrives. For the full picture, a technical SEO audit checks speed together with indexing and structure.

Why website speed is money

A Deloitte study commissioned by Google (2020, Europe and the US) found that a 0.1 s faster mobile site lifted retail conversions by 8.4% and average order value by 9.2%. In a Vodafone A/B test (2021) LCP improved by 31% and sales grew by 8%. If you pay for ads, a slow landing page burns part of the budget.

The usual causes of slowness and how we fix them are described on our website speed optimisation page: work at €40/h, the check is free. If you also need SEO, security and consent checked, the audit is €200. Get in touch.