It may be QXL. If you have SPICE graphics enabled (probably affects VNC too), check to see if you’re using QXL. Changing it from QXL to VMware’s VMVGA took idle CPU usage of 10%-30% to 3-7%.
Virt-Manager has a bug in it that won’t let you change it from QXL, so go to the command line and type in:
virsh edit domainname
Go down to to the video section and update the model type to be:
Shutdown and start the Windows guest again and you should be good to go.