I would love to parallelize a plugin script in Cura, the 3D print slicer. It does a bunch of embarrassingly parallel calculations, and could be made at least 16x faster for me. Because it's a plugin, though, it isn't pickle-able and multiprocessing doesn't work. I managed to make it work in a branch, but only on OSes that can fork processes. On windows, the plugin spawns multiple GUIs because importing the cura package apparently has the side effect of launching it...

If there wasn't the GIL, I could just create a thread pool and be done, and Cura could continue to be a delightful mess. :-)