A performance management system is provided that measures end user performance in a distributed computing environment. The system detects DOM updates caused by browser side activities, and identifies resource load requests introduced by a DOM update that request resource from third party sources. For such resource load requests, resource sensors are installed which detect the point in time when loading the resource was finished. This allows to measure load time for individual resource load requests requesting third party resources, and to assign the tracing and performance monitoring data describing those resource load requests to the tracing and performance monitoring data describing the browser side transaction execution that caused the third party resource loads.