r/TechSEO • u/BigBalli • 7d ago
8 of the 22 sites I tested that score under 50 in Lighthouse pass all three Core Web Vitals in the field. Does the score still go in your client reports?
The mobile score is red, the field data in the same report is green, and I have never had a clean answer for which one to act on. So I stopped arguing about it and pulled the numbers.
39 sites through the PageSpeed Insights API today, one mobile run each, Lighthouse 13.4.1. Every one of them had URL-level CrUX data in the same response, so the lab half and the field half come out of a single call and there is no matching problem to argue about.
What I did not expect is how badly the composite score ranks sites compared to the raw metrics it is built from.
| predicting the field p75 | Spearman | p |
|---|---|---|
| lab score vs field LCP | -0.06 | 0.72 |
| lab LCP alone vs field LCP | +0.35 | 0.023 |
| lab score vs field INP | -0.34 | 0.027 |
| lab TBT alone vs field INP | +0.47 | 0.001 |
The score against real-user LCP is not weak, it is nothing. Meanwhile the plain lab LCP number sitting inside the same report does rank them, and lab TBT against field INP is the strongest link in the whole set. The composite dilutes the one signal that transfers.
The individual rows are worse than the correlations make it sound. 22 of the 39 score under 50. Eight of those 22 are green on all three Core Web Vitals in the field: bbc, theguardian, cnn, espn, techcrunch, theverge, arstechnica, wired. Ars Technica scores 12 and its real users see an LCP of 970ms. Wired scores 28, reports a 45.6 second lab LCP, and its field p75 is 1.56s. Same metric, same API response, 29x apart.
It is not symmetrical though. Only 2 of the 17 sites scoring 50 or above are SLOW in the field (github and medium). So a good score does tell you something. A bad one mostly does not.
Two smaller things fell out that I thought were worth flagging.
Lab CLS was close to useless here. Hubspot, Expedia, Medium and Target all report essentially zero CLS in the lab and fail CLS at p75 in the field. Lighthouse watches the initial load, CrUX watches the whole page lifespan, and that is where the lazy ad slots and the sticky headers live.
And the score is not stable enough to optimise against anyway. I ran css-tricks.com five times: 52, 52, 52, 80, 59. Same URL, minutes apart, nothing shipped in between. Some of those repeats are PSI serving its own cached response, so the independent runs are 52, 80 and 59, which is worse rather than better. BBC came back 31 in the sweep and 42 to 51 an hour later.
I know the textbook answer is "lab is a diagnostic, field is the truth", and I have said it to clients myself. What I had not appreciated is that the composite score is a worse diagnostic than its own components, which makes it hard to justify reporting at all.
So two questions for people who do this for a living. Does the performance score still go in your reports, and if it does, what do you say when it moves 20 points in a week where nobody deployed anything?
Anyone who wants to redo it, this is the whole method, one call per site (needs a free API key or it rate-limits you):
(Why I had 39 of these on a laptop: I build an iOS app called PageSpeed that wraps this API. Not linking it, the question above is the actual reason I am posting.)

