The most important thing to measure is the user-experienced response time. If your app isn't ready until some assets have loaded and your UI is actually display

The most important thing to measure is the user-experienced response time. If your app isn't ready until some assets have loaded and your UI is actually displayed and usable, then you need to measure and instrument THAT. (domready is a poor but often usable proxy). A good APM tool will measure this as well for ajax requests by measuring how long it takes from click to the end of the callbacks executed after receiving the response (i.e. displaying the refreshed content).