The apps could be an issue, yes. Memory leaks are pretty easy to track down to a particular process. It's tracking down leaks in the application itself that can be tricky.
The paging thing...can vary from hardware to hardware. Measuring paging is pretty easy to track, too. But isolating performance issues down to specific causes isn't always easy. They are often pretty easy to spot. Something like CPU utilization, IO bottlenecks, excessive paging , etc. But sometimes, you get something where there seems to be no ready explanation for performance problems. I do engineering work, not operational work (unless the ops team needs to escalate to me). So when I deal with performance problems, its usually the not-so-obvious ones, that we are finding in the lab every once in a while. I think its kinda boring, actually. But that's part of the job sometimes.