Python's allocator makes use of the fact the GIL must be held to avoid any locking in the normal case for objects under 512 bytes. The introduction of pymalloc

Python's allocator makes use of the fact the GIL must be held to avoid any locking in the normal case for objects under 512 bytes. The introduction of pymalloc was a huge perf win at the time. For larger allocations, it's not so clear if a drop-in malloc would be such a win: many allocators just fall back to mmap() for big allocations and so I imagine their behaviour would be quite similar.