If you could release the Global Interpreter Lock while doing more CPU intensive calls to ClipperLib (union, difference, intersection, minkowski, offset, etc), it would allow Python apps to parallelize much better when using threading. I wrote a 3D printing slicer program in Python called Mandoline-Py ( https://github.com/revarbat/mandoline-py ), and ran headlong into the performance limitations of the GIL.

As best as I can tell, this should be just a matter of surrounding the CPU intensive code with Py_BEGIN_ALLOW_THREADS and Py_END_ALLOW_THREADS. (See https://docs.python.org/3/c-api/init.html#thread-state-and-the-global-interpreter-lock )