I have the same problem where i get only 15-19 FPS at max settings at KLAX. At other addon airports i get around 40-60 FPS depending on the scenery details. I also figured out that my problem is caused by GSX. the moment i disable GSX from the fsx menu, My FPS jump to 40+. Unfortunately with GSX disabled, all the jetways disappear in KLAX.This is by far the strangest thing ive seen after using tons of addon sceneries. Another reason might be that FSDT products are not well optimized like the FlightBeam and Aerosoft products which i use on daily basis. I always get 30+ FPS at max settings on other heavy addon airports like Mega Airport Frankfurt by Aerosoft. FSDT Sceneries are without any doubt the best looking but unfortunately they are badly optimized which causes serious lag even for high end systems.
My Specs -
i7-8750H @ 4Ghz
Nvidia gtx 1070 8gb
32gb Ram.