Don’t fall for this, MSFS is actually famous for not utilizing multi-core CPUs so if there is an application the 7700k is not at big disadvantage, then it is this. 18fps is ridiculous and not your hardware’s fault.
I have a very similar setup (7700k@4.8Ghz and an OC’d 3080), and performance/stability is much worse than before SU5 (now I actually cannot finish a flight due to various bugs and crashes since SU5), I could easily maintain 45 fps reprojection VR unless I was GPU bottlenecked.
If you want to troubleshoot it, make sure to assign at least 32Gb to virtual memory on your fastest drive.
Edit: just unfolded your later replies, my post is largely obsolete. 