I have 2 Windows VMs, one on cache drive and on on nvme (followed Space Invaders's guide), I normally only use the nvme but I left the cache one on for testing.  Both worked fine in 6.8.3 and both are extremely slow after upgrade.  I noticed in task manager the "System Interrupts" was using over 60% of my CPU randomly on both VMs.  It is setup as a gaming VM using Nvidia 1660 TI with 16G of ram, I thought that the GPU passthrough may be the problem so I tried VNC and it was still slow.