The work you're doing is just out of this world! That being said, a proper, well made Luton scenery would be an amazing addition to the simulator. (I have way too many photos of LTN so I could always send some and make use of them)
Also happens to me, when looking away from the airport I get 60 fps, but as I look at it it drops below 30. VRAM crashes happen often as well, so I have to use 1K textures
Looking forward to a fix/patch fairly soon because it's a lovely airport to fly to
Igor