OK this is sort of resolved…of course as usual, I can spend hours upon hours trying everything I can think of with no luck, but as soon as I ask for help, I solve it myself.
I found this thread Low GPU Usage / half Performance of Baremetal! (rtx3090) - pls :( - VM Engine (KVM) - Unraid which suggests adding this line “” to your XML. I did that, and suddenly I’m getting baremetal-like performance again, in the VM.
I don’t know for sure what this line does though. That forum post links to another discussion here on L1T that I’m reading through now KVM parameters to optimize Hyper-V with AMD processors - #3 by vic
Someone there mentioned that that really shouldn’t have made a difference. They mentioned that maybe Hyper-V was enabled in the guest which it shouldn’t have. I’m going to try removing that line and then check to make sure Hyper-V is not enabled on my guest and see if that makes a difference.
If anyone knows of any downsides or caveats to adding this line, please let me know. So far it seems to have made all the difference!