> What parts of Future are performance critical? Maybe it is worth to implement in C only the most critical code.

Basically everything.  Contrary to @contextmanager, Futures are the building blocks of asyncio, so instantiation + awaiting on them + setting results must be fast.

To cover instantiation, I want to add a freelist for Futures, so this basically requires them to be implemented in C (and it's not a lot of C code actually).