The fetchLater() API queues an HTTP request that the browser fires on bfcache entry, page hide, or a timeout, plugging the reliability gaps that make sendBeacon drop 5-15% of your RUM data on mobile.
Break down Time to First Byte into named backend phases with the Server-Timing header. Includes syntax, CORS gotchas, Node/Workers/nginx examples, and RUM integration.
INP became a Core Web Vital in March 2024, and most of us reached for the same tools we'd been using for FID. For the trickier regressions you'll stare at clean traces and lose a week.