Why Your Core Web Vitals Fix Isn't in CrUX Yet (28-Day Window)

You shipped the fix on Tuesday. Hero images are compressed, the heavy tag is gone, and Lighthouse on mobile looks healthier than Monday's run. On Thursday the account manager forwards a Search Console screenshot: the URL group is still Needs improvement. In most cases the change is real. CrUX has not finished rolling the old sessions out of its window yet.

CrUX is a 28-day rolling average of real Chrome sessions, not a lab run taken on ship day. Sessions collected before your fix stay inside the window until they age out, so a green lab run can sit next to amber field bands for days or weeks.

What to monitor while field data catches up:

• Baseline lab LCP, INP, and CLS on mobile and desktop before you change anything

• Scheduled lab runs after ship so before/after is stored, not remembered

• Budgets and alerts on lab vitals so regressions during the CrUX wait still notify someone

• Weekly field checks with collection period dates noted in the client report

Close the engineering ticket when lab verification passes. Keep a separate field-watch item until Search Console moves into the agreed band. One sentence in the retainer report saves a week-one reopening: field CrUX remains a 28-day rolling average; lab trends below are same-week verification.

Read more: why your Core Web Vitals fix is not in CrUX yet (28-day window)