OK. I'll give this my best shot. I recently reinstalled FSX due to hardware changes. I wanted to see how it performed. Only SP1 and 2 are installed. So, after reading your post, first, I reset all settings to default, at the main Settings screen. Then, all the sliders were maxed out. Then, the only thing I changed under customize, was the resolution. I changed it to 1920x1080x32. I noted the target FPS was set to 30 by default. I grabbed a cessna and jumped into a flight.
I averaged 20 fps, and it still looked like ***. I was even flying in a remote area w/o any real traffic figured in yet.
I run an i7-2600k at 4.6ghz, 1.32v
2 EVGA 3GB GTX 580s in SLI - 772mhz default clock
16GB G.Skill Raptor X - 1866 9-10-9-28
FSX installed to a 7200rpm WD Caviar Black 2 TB (just for reference, no effect on fps)
I used to have all those hyper-realistic mods, but haven't bothered installing them yet. Are you running a standard install or do you have some customizations? What resolution do you use? Do you want me to try a different resolution to see how it effects the fps?