I agree the start-up time is important. There is something that is related. ABCMeta is currently implemented in Python.
This makes it slow, creation of an ABC is 2x slower than creation of a normal class.
However, ABCs are used by many medium and large size projects.
Also, both abc and _collections_abc are imported at start-up (in particular importlib uses several ABCs, os also needs them for environments).
Finally, all generics in typing module and user-defined generic types are ABCs (to allow interoperability with collections.abc).

My idea is to re-implement ABCMeta (and ingredients it depends on, like WeakSet) in C.
I didn't find such proposal on b.p.o., I have two questions:
* Are there some potential problems with this idea (except that it may take some time and effort)?
* Is it something worth doing as an optimization?
(If answers are no and yes, then maybe I would spend part of my vacation in August on it.)