I'm not sure this is the conventional use of the phrase "fast path."

But anyway, the idea is usually that the Python code calls out to the framework with operations that are in some way "large," and so the overhead is not so significant.

Python probably doesn't have to do any marshaling, hypothetically the framework could just return an object that represents a pointer. Then the python code sends that pointer to another framework method.

I think you mean critical path (which is ironically usually the slowest path). A fast path is usually a hardcoded shortcut you can take for select cases.