FS2020 is using DX11 so what you are seeing with your cores is mostly accurate.
It took a bit of stuffing around but I'm running it on a VM with 4c/8t of a 3700x isolated and passed through and am seeing exactly the same performance through the VM as baremetal. Averaging 40fps in the air with most settings on Ultra and a 1660Ti. Performance takes a fair performance hit on the ground but this is expected. i440 performs slightly better for me than Q35 however does introduce slight stuttering at times. Using hugepages for RAM also seems to help and the more the better (more than 16gb).
This helped in my case.
https://www.reddit.com/r/MicrosoftFlightSim/comments/ig1mwz/running_msfs2020_under_qemu_utilizes_cpu_poorly/
Happy to share my XML if you want to have a look but there's nothing too special about it.