I really don't care about the performance of Windows. I
Up to a point, I don't either. But the performance is so bad under KVM that I cannot even successfully run Windows Update, it takes too long to search for updates and eventually times out (other network-related things work so I know it's not my network connection). This does not happen under VirtualBox.
I might at some point look into alternate drivers, but it's easier to just install VirtualBox. But VB is not free software, so I might be motivated enough to try making Windows-under-KVM work better at some point (and I haven't missed the obvious point that Windows isn't free software either, I just have reasons why I have to use it).