My first areas to check would be…
-
Power supply. Since things are working, but slowly, wondering if the GPU is getting enough power. Can you try reconfiguring the PCIe power cable setup going to the card? (Different ports on the power supply, if it’s modular? Or just swap around the PCIe cables into the adapter.)
-
Flatten the system: Back up your data and reinstall Windows from scratch. I’ve had horrid MSFS performance problems solved by just rebuilding the OS and MSFS install from scratch.
Something’s definitely awry, given the 2D perf is an issue too. Maybe use a utility to see how much power is going to the GPU?