That’s because even on the main menu, the sim is rendering full quality models in the background. I’m not sure that you noticed but behind the menu UI, you should see there’s a hangar fully rendered, with the latest aircraft sitting inside the hangar, fully rendered both the exterior and the interior as well.
So even when you’re just on the main menu, the sim still continuously rendering this.
In terms of your CPU, have you tried using the DEV Mode display FPS function? It will always tell you where the bottleneck is. I also had a few stutters and low FPS on my Intel i9 CPU. But when I followed this guide, it actually solved a lot of my FPS and stutter problem.
I believe there should be a comparable function in your AMD bios that can disable the same virtualisation feature.