I fly in and out of O'hare a lot. It seems that for me personally the performance tanks significantly with DD's City and DD's MDW. but it's also not stellar if I have both of DD's sceneries disabled. Anyone else struggling there? I am not sure if ORD isn't well optimized or what, but it's super choppy for me. 

My opinion... Gsx is causing worse frame rates.  My FSDT ord performance is notably decreased after gsx and i have wierd issues with the gsx menu being stuck on loading. Add AIG and your down to 25 fps.  I do not have either dd scenery.  I have also noticed that Chicago, like LA and NYC, has always been hard on frames.