I spent the weekend trying to diagnose the problems and I may have found something. These are the things I've tried with mixed results:
Moved everything off my cache pool (2 Samsung EVO SSDs), repartitioned to 1MB alignment, moved everything back (my vdisks are on the cache pool)
Updated virtio windows drivers to latest (0.1.190-1)
Updated VM Machine to i440fx-5.1 (was i440fx-4.2)
Changed VM network model to virtio-net (was virtio)
Reduced RAM to 16384 MB (was 32768 MB)
Moved Windows 10 VM vdisk to separate SSD in unassigned devices
Changed VM to use /dev/urandom for RNG
I have two Windows 10 VMs and only one is having problems. The last change I made this morning looks to have resolved the issue for me. In the Windows 10 VM that is slow, I went into Windows Features and disabled Hyper-V, Windows Hypervisor Platform and Windows Subsystem for Linux. After the required reboot, I no longer saw all cores hitting 100% with 50-80% from system processes. I also saw a dramatic change in the interrupt processing on the cores dedicated to that VM from Unraid's perspective. My next step is to turn Windows Subsystem for Linux back on since I do need it for this VM.
Hope this info helps some of you experiencing slowness.