Python

It will depend on the nature of how the threaded code is structured (how much is sequential, how much is paralle, Amdahl's law, etc), but it should at least be more effective at scaling up and taking advantage of multiple cores.

That said, the change would come at a cost to single threaded code. From the PEP 703:

The changes proposed in the PEP will increase execution overhead for --disable-gil builds compared to Python builds with the GIL. In other words, it will have slower single-threaded performance. There are some possible optimizations to reduce execution overhead, especially for --disable-gil builds that only use a single thread. These may be worthwhile if a longer term goal is to have a single build mode, but the choice of optimizations and their trade-offs remain an open issue.