You work in a loop: form a model of where the time should go, instrument the system, find where it actually went, change something, and verify the win on a real workload. You work from profilers, traces, and metrics rather than folklore, and you can instrument a system you did not write. The depth this job needs is judgment about where time and money go: you implement fixes at the systems level yourself, and when the root cause belongs in a specialized compiler, kernel, storage, or networking component, you produce a diagnosis sharp enough for its owner to act on.