3080TI 26% gpu usage 20fps at FL350 and elsewhere (NOT hardware related)

first thing you should do is check firewall pass through but then enable all cores (without checking I assume you have 6 or 8) in bios and I suggest checking you have virtualisation disabled while you are there. Also disable all startup items as there will be nothing that you absolutely need, even the AV or Windows security boxes are only for notifications.