The language and operating system in use will have a big influence on this- what are they?

E.g. for C programs I use sampling-based profiling (like Mac makes super easy with the “Time Profiler” Instrument) to find the bottleneck(s) and then drill down to see associated code sections. Different languages and OSs have different tools for identifying bottlenecks.

If performance is network-bound, that’s a very different set of considerations (with which I have no experience)