Why does PageSpeed Insights show “Core Web Vitals Assessment: Failed” while the performance score is high (green circles)? And how can another website have “Core Web Vitals Assessment: Passed” but show a low performance score (red circle)?

It is one of the questions I am asked most often, and the answer is simple once you know that PageSpeed Insights is really two reports on one screen. The Core Web Vitals assessment at the top uses real visitor data, while the performance score underneath comes from a single simulated test. They measure different things, so they can easily disagree.

1. Core Web Vitals Assessment

The Core Web Vitals assessment focuses on three user experience metrics, measured from real Chrome users who visited your page (or your whole site) over the last 28 days:

Largest Contentful Paint (LCP) measures loading. Good is 2.5 seconds or less.
Interaction to Next Paint (INP) measures how quickly the page responds when someone taps or clicks. Good is 200 milliseconds or less. INP replaced First Input Delay (FID) in March 2024.
Cumulative Layout Shift (CLS) measures visual stability. Good is 0.1 or less.

Google judges each metric at the 75th percentile, meaning three out of four visits must be within the “good” limit. All three must be good for the assessment to pass:

Passed means LCP, INP and CLS are all good for real visitors.
Failed means at least one of them is not.

2. Performance Score

The performance score (0 to 100) is a Lighthouse lab test. PageSpeed Insights loads your page once, on a simulated mid-range phone with a throttled connection, and scores it on metrics such as First Contentful Paint, Largest Contentful Paint, Total Blocking Time, Cumulative Layout Shift and Speed Index.

Because it is one test in controlled conditions, it is useful for finding problems, but it does not show what your real visitors experience. It also cannot measure INP, because no one is interacting with the page during the test.

Why Discrepancies Happen

Case 1: “Core Web Vitals Assessment: Failed” but a green performance score

The lab test went well, but real visitors are having a worse time. Common reasons:

Slow interactions (INP). Heavy JavaScript, chat widgets, cookie banners or tracking scripts make the page slow to respond to taps. The lab test never taps anything, so it cannot see this.
Layout shifts after loading. Adverts, late-loading images or pop-ups that move content while people scroll are counted in the field data but often missed in a short lab test.
Real devices and connections. Many visitors use older phones or poor mobile signal, so pages load more slowly than in the test.
Old data. Field data covers the last 28 days, so recent improvements take up to four weeks to show.
Site-wide data. If a page has too few visits of its own, PageSpeed Insights shows data for your whole site (the “origin”), so slow pages elsewhere can fail the assessment for a fast page.

Case 2: “Core Web Vitals Assessment: Passed” but a red performance score

Real visitors are having a good experience, but the strict lab test still finds problems. This often happens when most visitors use fast devices or return to the site with files already cached, or when the lab test is affected by large scripts that real users barely notice.

Which result matters for SEO?

Google uses the real visitor data, the Core Web Vitals assessment, as part of its page experience signals. The lab performance score is not used for rankings. So a failed assessment with a green score is the one to fix, while a red score with a passed assessment is a lower priority.

How to fix “Core Web Vitals Assessment: Failed”

Find the failing metric. Check which of LCP, INP or CLS is red in the field data at the top of the report, and use the Core Web Vitals report in Google Search Console to see which groups of pages are affected.
Fix LCP by compressing and correctly sizing the main image, preloading it, using fast hosting and caching, and removing render-blocking CSS and JavaScript.
Fix INP by cutting unused JavaScript, delaying third-party scripts until after the page loads, and removing plugins you do not need.
Fix CLS by giving images, videos and adverts fixed dimensions and stopping banners from pushing content down. My guide on fixing the “CLS issue: more than 0.25 (mobile)” warning walks through this step by step.
Wait for the data. After fixing, use “Validate fix” in Search Console and allow up to 28 days for the field data to update.

On WordPress sites the usual causes are heavy page builders, too many plugins and unoptimised images. If you would rather have this handled for you, my WordPress SEO consultancy includes Core Web Vitals fixes.

In Summary

The Core Web Vitals assessment uses real visitor data for three metrics (LCP, INP and CLS).
The performance score is a single simulated lab test using a broader set of metrics.
A page can score highly in the lab and still fail with real visitors, and Google ranks on the real visitor data.

Why does PageSpeed Insights show 'Core Web Vitals Assessment Failed' while the performance score is high

Failing Core Web Vitals on a site you rely on? Send me your URL for a free check.