FWIW: I think David's cited behavior proves that the GIL is de facto a scheduler.  And, in case you missed it, scheduling is a hard problem, and not a solved problem.  There are increasingly complicated schedulers with new approaches and heuristics.  They're getting better and better... as well as more and more complex.  BFS is an example of that trend from ten years ago.  But the Linux kernel has been shy about merging it, not sure why (technical deficiency? licensing problem? personality conflict? the name?).

I think Python's current thread scheduling approach is almost certainly too simple.  My suspicion is that we should have a much more elaborate scheduler--which hopefully would fix most (but not all!) of these sorts of pathological performance regressions.  But that's going to be a big patch, and it's going to need a champion, and that champion would have to be more educated about it than I am, so I don't think it's gonna be me.