Growing pains for typing in Python

Sure, if you're creating your own baroque classes and trying to do all kinds of things to them, this will be a problem for you. The answer is presumably not to do that, and also that it's pretty rare.

Also, presumably there are ways to work around the issues you describe - they certainly don't prevent C# and Java from having absurd numbers of libraries. Don't slow them down in the slightest as far as I can see.

I would also add that the world at large disagrees about large vs small projects - Larger Python projects are the ones driving use of type checking.