I asked why on StackOverflow, and an user seemed to find the reason. The problem for him/her is in `update_one_slot()`. 

`dict` implements directly `__contains__()` and `__getitem__()`. Usually, `sq_contains` and `mp_subscript` are wrapped to implement `__contains__()` and `__getitem__()`, but this way `dict` is a little faster.

The problem is that `update_one_slot()` searches for the wrappers. If it does not find them, it does not inherit the `__contains__()` and `__getitem__()` of the class, but create a `__contains__()` and `__getitem__()` functions that do an MRO search and call the superclass method. This is why `__contains__()` and `__getitem__()` of `dict` subclasses are slower.


Is it possible to modify `update_one_slot()` so that, if no wrapper is found, the explicit implementation is inherited?

SO answer: https://stackoverflow.com/a/59914459/1763602