@aroos: Thank you for the tm.log file. At least no relevant issues are found. Your GPU seems to offer only 2 GB of memory. The huge drop you see between the different AA settings is probably related to a GPU memory issue, e.g. Aerofly FS probably needs more than 2GB and if we need to access memory not on the GPU things slow down considerably. We haven't optimized Aerofly FS for those scenarios. High quality shadows for example might already consume almost 512 MB a quarter of your available GPU memory. And also keep in mind ORBX KPSP and LOWI by itself are very detailed and will consume over 2.5GB with Medium quality.
As a test, can you fly over a region with low buildings and 3D objects. Do you see the huge difference there as well?