The big difference from Python is probably having to use a real tracing GC instead of automatic reference counting. For a single-threaded program, refcounts are beneficial in multiple ways, being fairly cheap, having a smooth performance profile, maintaining low resident set size, and providing deterministic freeing.

But because of the way cache coherency for shared, mutated memory works, parallel refcounting is slow as molasses and will always remain so.

I think Ruby has always used a tracing GC, but it also still has a GIL for some reason?