In 3.6.0, invocation of isinstance calls typing._ProtocolMeta._get_protocol_attrs.
This creates a set of all attributes in all base classes, loops on these attributes to check they exist, and discards the set. It is very slow.

My program uses isinstance to allow for flexibility in parameter types in certain key functions. I realize that using isinstance is frowned upon, but it seems to make sense in my case.

As a result, >95% of its run-time is inside typing._ProtocolMeta._get_protocol_attrs (!).

I have created a simple wrapper around isinstance which caches its result with a Dict[Tuple[type, type], bool]. This solved the performance problem, but introduced a different problem - type checking.

I use mypy and type annotations, and my code cleanly type-checks (with the occasional # type: ignore). If I switch to using my own isinstance function, then mypy's type inference no longer treats it as special, so it starts complaining about all uses of values protected by if isinstance(value, SomeType): ...

I propose that either the private typing._ProtocolMeta.__subclasscheck__ (which invokes _get_protocol_attrs), or the public isinstance, would be modified to cache their results.

I can create a PR for either approach, if this is acceptable.