What have you found are the most useful approaches to collecting the representative issues in a way you can reuse and test? I haven’t worked as much in the web space.

A combination of: 1) RUM, 2) Field tracing, 3) Local tracing with throttling (CPU, Network), 4) Focusing either on known problems, or if I'm on less certain territory I will do more vetting to make sure I'm chasing something real. I'll minimize the time spent on issues that I can only catch in one trace, but sometimes it's also all you get so I'll time box that work carefully. It's more of an art than a science.