Here's further tests to check how fast different class checkers are:

```python from typing import MutableMapping

class Test2(MutableMapping): def init(self, args, kwargs): self.data = dict(args, **kwargs)

def __getitem__(self, key):
    pass

def __setitem__(self, key, value):
    pass

def __delitem__(self, key):
    pass

def __iter__(self):
    pass

def __len__(self):
    pass

b = Test2()

%timeit issubclass(type(b), MutableMapping) 711 ns ± 5.33 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)

%timeit isinstance(b, MutableMapping) 853 ns ± 6.29 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)

If you want to get really fast you can check for one of the required attributes MutableMapping has

%timeit hasattr(b, "update") 82.6 ns ± 0.181 ns per loop (mean ± std. dev. of 7 runs, 10000000 loops each) ```

isinstance is rather slow as can be seen. Considering just doing dict(b) takes about 200ns which is basically what the original implementation was it doesn't feel that good to add a check that adds 800ns of wait time.