Hm okay. I passed 14 of 16 threads of my i9 9900k to MacOS and it was barely usable. After passing through a low end AMD gpu, everything was really fast (faster than the 8 core iMac Pro, benchmark wise, iirc).
Did you use KVM accelerator? I think there’s also another flag to use the native cpu without emulation that you can use in your case. But I’m a newb at qemu