The GILectomy project was able to maintain thread safety on refcounting while (mostly) preserving performance by basically using some clever flags in the refcou

The GILectomy project was able to maintain thread safety on refcounting while (mostly) preserving performance by basically using some clever flags in the refcount to signal specific lifetimes/ownership (simplification for brevity). So you have good performance when the thread owns a reference while preserving safety for shared objects. The smaller problem is this does at a small amount of overhead even to single-threaded performance. The gilectomy project improved performance elsewhere so net performance is close to the same. The bigger problem would be integrating this strategy with all the libraries that rely on existing GIL behavior.