https://www.rum-core-web-vitals.com/custom-metrics-business-impact-tracking/user-timing-api-marks-and-measures/
User Timing API: Marks & Measures | Real-User Monitoring
Define custom metrics with performance.mark() and performance.measure(), observe them with PerformanceObserver, and ship app-specific timings to RUM at p75.
user timing apimarksmeasuresrealmonitoring