Lab tests don't match real user experience. Monitor in production.
Real User Monitoring (RUM): Collect metrics from actual users.
Tools: Google Analytics, Lighthouse CI, custom tracking.
What to track:
- Core Web Vitals
- Time to interactive
- JavaScript errors
- API latency
Segment by:
- Device type
- Connection speed
- Geography
Interview tip: Mention monitoring when discussing performance. It shows production experience.