The idea is that creating ABCs is approximately twice slower than normal classes. Plus there are smaller penalties for various operations with ABCs. This mostly influences the Python interpreter start-up time (because of extensive use of ABCs in importlib), and start-up times of programs that extensively use ABCs.

The situation can be improved by rewriting ABCMeta in C. I have a working implementation, but it is far form being ready and still needs some polishing and optimizations (in particular _abc_cache and friends).

Already at this stage I have one question (I will add more when they appear while I am finishing the implementation): is it OK to make _abc_cache, _abc_negative_cache, _abc_negative_cache_version, and _abc_registry read-only? The point is that I want to prohibit something like this:

    MyABC._abc_cache = "Surprise on updating the cache!"

thus avoiding many PySet_Check(...) calls. These attributes are not documented and start with underscore.