hello All Simmer
I have the same problem frame rate in fsx
What I learn in past 4 years is no matter what comp spec do you have is never enough to play fsx on max setting!
Microsoft when release FSx doesn't care about all the addons we will install on it after .
I did one test 2 years ago I install fsx without any addons and I try to play on high setting and this was working just  fine about 30 fps all the times I think is enough.
when we start install new airport and aircraft FPS drop down drastically and we need to minimize the setting to play the game.
Today I buy new video card nvidia geforce 480 gtx just to play fsx with high setting and frame rate better  then 8 fps on Egll mega airport heathrow.
Unfortunately my power supply is to weak to give enough power my graphic card so tomorrow I will need to buy 850 watt monster
and hopefully I will be able to play FSX on setting better then medium low :)

let you know how's  it going .
regards
Chris