CPython is so hopelessly slow, I wouldn't care about 10%. For most of the stuff written in Python, users don't really care about speed. The impact won't be on

CPython is so hopelessly slow, I wouldn't care about 10%. For most of the stuff written in Python, users don't really care about speed. The impact won't be on users / Python programmers who don't develop native extensions. It will suck for people who had a painful workaround for Pythons crappy parallelism already, but now will have to have two workarounds for different kinds of brokenness. It still pays off to make these native extensions, however their authors will create a lot of problems for the clueless majority of Python users, which will like end up in some magical "workarounds" for problems introduced by this change very few people understand. This will result in more cargo cult in the community that's already on the witch hunt.