London and any other large city kills my FPS also, as soon as I leave the area I'm back at 35ish FPS. Admittedly, I do have most quality sliders up to the right, a number are maxed. Level of detail radius maxed, Tessellation factor maxed, Mesh resolution 1m, texture resolution 50cm.
If I am flying a 3rd party complex aircraft the hit is much worse.
Prepar3D 2.2
FTX Global
FTX Vector
Affinity mask set to utilise all cores.
i7 4770K (oc'ed to 4.2Ghz)
AMD 7950 3GB graphics card
8GB RAM
Performance in general is fantastic though and I'm not complaining. I need to find a nice balance really, as flying over cities makes the sim unplayable at my current settings.