Lighthouse or PageSpeed lab runs look healthy, but Search Console's Core Web Vitals report or CrUX field data shows URL groups rated Poor or Needs improvement for LCP, INP or CLS. Google says Core Web Vitals are used by its ranking systems, and its report is built on real-user field data at the 75th percentile. A team optimising lab scores can believe the job is done while the data Google reports still fails.
Why does core web vitals pass in lab tests but fail in field data matter for AI search?
Google says Core Web Vitals are used by its ranking systems, and its report is built on real-user field data at the 75th percentile. A team optimising lab scores can believe the job is done while the data Google reports still fails.
How do you fix it?
Diagnose on the failing templates with real-user monitoring or CrUX breakdowns, reproduce under slower devices and networks, then fix the specific metric (image and server delivery for LCP, long tasks for INP, reserved space for CLS). Validate in Search Console after the 28-day window.
How do you find it on your site?
- Open Search Console, Experience, Core Web Vitals, and list mobile and desktop URL groups rated Poor or Needs improvement.
- For an example URL in each failing group, run PageSpeed Insights and compare the field data section with the lab section.
- Flag templates where lab metrics pass Google's thresholds but field LCP, INP or CLS fail at the 75th percentile.
- Record the affected template, metric, device and 75th percentile value for the fix list.
Which ranking and citation factors does it touch?
Written for the Page experience system page (evidence: Confirmed).
What is the impact?
Direct but small ranking effect where relevance is close; larger user experience effect. Estimate only.
What is the evidence?
Google's Understanding page experience documentation and Core Web Vitals documentation; Search Console Core Web Vitals report help.
Sources
Frequently asked questions about core web vitals pass in lab tests but fail in field data
What does "Core Web Vitals pass in lab tests but fail in field data" mean?
Lighthouse or PageSpeed lab runs look healthy, but Search Console's Core Web Vitals report or CrUX field data shows URL groups rated Poor or Needs improvement for LCP, INP or CLS.
How do you fix core Web Vitals pass in lab tests but fail in field data?
Diagnose on the failing templates with real-user monitoring or CrUX breakdowns, reproduce under slower devices and networks, then fix the specific metric (image and server delivery for LCP, long tasks for INP, reserved space for CLS). Validate in Search Console after the 28-day window.