I do wonder, on a local filesystem, how much of the time is statting paths vs. reading the file contents vs. unmarshaling code objects. (Top-level code also runs when a module is imported, but the cost of that is of course highly module-dependent.)

Maybe you could take the stat timings, the read timings (both from strace) and somehow instrument Python to output timing for unmarshalling code (or just instrument everything in python).

Either way, at least on my system with cached file attributes, python can startup in 10ms, so it's not clear whether you truly need to optimize much more than that (by identifying remaining bits to optimize), versus solving the problem another way (not statting 500 files, most of which don't exist, every time you start up).