The most significant impact of this change is that python threads can run at the same time. Before, calls to C API were essentially cooperative multithreading yield points - even if you have 100 python threads, they can't run concurrently under the GIL. 99 are blocked waiting on the GIL.

C extensions have always been able to use real threading. But now there is no GIL to synchronize with the python interpreter on ingress/egress from the extension.