Learn the three current Core Web Vitals metrics: Largest Contentful Paint (LCP) good threshold is under 2.5s, Interaction to Next Paint (INP) good threshold is under 200ms, Cumulative Layout Shift (CLS) good threshold is under 0.1
Understand the p75 rule: a page passes a metric only if at least 75% of real-user sessions are in the 'good' range — the 75th percentile value must be at or below the good threshold
Retrieve p75 values from the CrUX API or Search Console Core Web Vitals report, not from Lighthouse lab scores which measure a single synthetic run
When a URL has insufficient real-user data, the CrUX API falls back to URL-group level, then to origin level — identify which granularity is being returned via the 'record.key' in the API response
Use the Search Console Core Web Vitals report to identify which URL groups (grouped by similar URL patterns) are failing, then investigate representative URLs with PageSpeed Insights
Track pass rate trends over time using the CrUX BigQuery dataset (bigquery-public-data.chrome-ux-report) rather than spot checks to understand regressions
Known gotchas
INP replaced FID (First Input Delay) as a Core Web Vital; tools or dashboards still reporting FID are using outdated data and FID is no longer a ranking signal
Lighthouse lab scores are not the same as CrUX field data — a page can score 95 in Lighthouse and still fail Core Web Vitals if real users on slow connections or devices experience poor metrics
The p75 rule means even a small tail of poor experiences can cause a page to fail; optimizing median performance without addressing outliers may not move the needle on CrUX pass status
Give your agent this knowledge — and 200+ more routes
One MCP install gives any agent live access to the full route map, with trust scores updated by agent consensus:
claude mcp add --transport http waymark https://mcp.waymark.network/mcp