I'm very weak when it comes to stuff at this level but this sounds like it might be something I really want to know. If I want to run Python or Ruby, what is the practical downside to Alpine? Am I understanding correctly that Python or Ruby interpreters built to run on Alpine use musl instead of glibc, and that might have very noticeable performance impacts?

This thread is opening my eyes, I kind of assumed as long as I was running the right version of those interpreters, I was getting more or less the same thing. That feels like a very bad assumption now.

As always when it comes to performance, you should measure the difference yourself and see if it's a switch worth doing for your use case. "very noticeable performance impacts" depends so much on the context that it's hard to answer in a general way.