It would definitely be faster on qemu. I am basing this on the fact that running windows in qemu is night and day difference compared to vitualbox. If you run Windows in qemu in full-screen you can’t even tell you are in a VM. It’s that responsive.
Also I am running OSX successfully on QEMU and performance is very good. Again I am basing my initial assessment based on my Windows experience so I can’t really compare how OSX runs in vbox, but I can only say it runs pretty good in qemu.
You might also be interested to see that there is another option to run XCode in Linux without installing OSX in a VM:
This is news to me too, and I’ll definitely give it a spin as soon as I get some time for it, because I only installed OSX to be able to use XCode.
Btw, I’ve followed this tutorial to setup my macOS instance: