This is a backport of a patch in SPIRV-Tools that will allow faster parallel compilation. See https://github.com/KhronosGroup/SPIRV-Tools/issues/5802 for the original report. The issue is that SPIRV tools used a stringstream to convert an uint32_t to a string when optimizing shaders. stdlib provided by the platform would set a locale and would mutex globally, making parallel compilation fully lock. Compilation lead times were as fast as single threaded compilation. Pull Request: https://projects.blender.org/blender/blender/pulls/127564