Show CrUX field data alongside lab Web Vitals#4794
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A lab run measures one device, one network, one location — often a fast, cached, well-connected one. That makes the lab Web Vitals look healthier than what real users actually experience, and a regression real users already feel can stay invisible in the lab numbers. sitespeed.io already collects CrUX field data when configured, but it lived on a separate tab, so nobody compared the two. This puts the CrUX field p75 directly under the lab median in the Google Web Vitals card for TTFB, FCP, LCP, CLS and INP, graded against the same Good/Needs-improvement/Poor thresholds. The field line is matched to the device you tested (mobile run → PHONE, desktop → DESKTOP, falling back to ALL when the matching form factor wasn't collected) and always labels which form factor it used, so the two numbers are never silently mismatched. When the lab grades a metric better than the field does, the card flags it as "lab is more optimistic than real users" — the cue that your test conditions are hiding a real-world regression. Co-authored-by: Claude noreply@anthropic.com
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A lab run measures one device, one network, one location — often a fast,
cached, well-connected one. That makes the lab Web Vitals look healthier than
what real users actually experience, and a regression real users already feel
can stay invisible in the lab numbers. sitespeed.io already collects CrUX field
data when configured, but it lived on a separate tab, so nobody compared the two.
This puts the CrUX field p75 directly under the lab median in the Google Web
Vitals card for TTFB, FCP, LCP, CLS and INP, graded against the same
Good/Needs-improvement/Poor thresholds. The field line is matched to the device
you tested (mobile run → PHONE, desktop → DESKTOP, falling back to ALL when the
matching form factor wasn't collected) and always labels which form factor it
used, so the two numbers are never silently mismatched. When the lab grades a
metric better than the field does, the card flags it as "lab is more optimistic
than real users" — the cue that your test conditions are hiding a real-world
regression.
Co-authored-by: Claude noreply@anthropic.com