Set up field-data monitoring in 4 steps

Set up field-data monitoring in 4 steps
If you only test in the lab, you're blind to p75 reality. Stand up real-user measurement before you optimize.

— Step 1: Add the web-vitals JS lib; capture LCP, INP, CLS, TTFB with attribution.
— Step 2: Send to an endpoint with device, connection, and route as dimensions.
— Step 3: Compute p75 per route per day — a single average hides the failing tail.
— Step 4: Cross-check against the free CrUX History API monthly for ground truth.

Why p75: Google grades at the 75th percentile — your median can pass while 1 in 4 users fails.

Takeaway: track p75 per route, not a site-wide mean — that's the number Google scores.