Same problem. FPS is decent not great, but after some amount of time it drops to being totally unplayable. I have the lower resolution textures installed and running on what would be considered medium settings in MSFS, with a high speced computer. The one thing that may help tracking this down, is for me, shifting to an external view fixes it temporarily. If I hit F2 and wait for the sim to catchup, it fixes my FPS, both in cockpit and external view, but it is very temporary.