Just a quick update. I have refactored importlib in the cpython repo to allow for implementing bits of importlib.__import__() and importlib._gcd_import() in C. This means that the built-in __import__() is now calling importlib underneath the covers. Eventually what is in importlib.__import__() and _gcd_import() directly will be translated into the equivalent C code. This will speed up accessing sys.modules. After that whatever is deemed on the critical path and worth rewriting in C can be done function by function. That, though, should wait for Python-level profiling and optimization before wasting one's time.