I have not run a Windows VM in Qubes, and I've been using it full time for only about a week now, but: performance has not been an issue with up to at least 8. This is with an NVMe drive, Ryzen 5600G, and 32GB of RAM.
Usability, however, is a bit wonky, but that's the trade-off for security. I'm sure my relative inexperience with it is at play there, as well.