> A coroutine detection is a relatively slow check.
> I don't think we need to do it in `functools.lru_cache`.

Wouldn't a coroutine check only happen during decoration time? To successfully solve this easily and efficiently, we only really need to wrap the coroutine with `asyncio.ensure_future` if the decorated function is a coroutine, and it will only happen when a result comes back from the decorated function which would have minimal impact.

Of course, I don't know much about the internals of `lru_cache` so my assumptions could be wrong. I should familiar myself with the implementation and figure out how doable it would be.