Found a possible solution in the proxmox forum. Basically posted the same there, but since it took me a while to actually find it I'm duplicating it here.
https://forum.proxmox.com/threads/high-cpu-load-for-windows-10-guests-when-idle.44531/
The hpet setting seems to fix it for me. After setting it to yes, everything seems back to pre-1803 levels.
edit: fixed link