Use any online PHP profilers like these ones:
Magento has a built-in profiler but its features are limited.
Once you hook it up with a profiler, analyze callgraphs to see if any function calls are slow.
You can always try and do a 3rd-party extension audit. From my experience, 90% of all performance issues come from some custom plugin. Try turning them all off and see if it makes any difference. If it does, turn them back on one by one and find that abuser(s).