I'd say that 18 fps with a complex airplane and full AI at KLAX while approaching sounds very reasonable with your system
I would completely agree with you, Umberto!
Like I wrote above, that's a similar number I am getting at all giant airports around my FS world.
This is the resultant of an old PC, an old OS and tones of addons I personally need to use for the way of simming which satisfies me.
And I can control an aircraft properly even when my FPS occasionally drop to around 10 FPS.
Not that I like it of course, but on a scenic approach to a large international airport, with well-developed cumulus clouds lit by the setting sun, lots of dynamic AI aircraft both boarding, taxiing, departin and arriving, moving ground vehicles, AES stuff waiting for me, 100% autogen trees and buildings, HQ ground textures (GEPII), UT detailed land features, a complex 19m or 32m mesh, ASE, RC4 and FS2Crew working in the background, FSNav showing my flightplan, and all that seen from my high quality virtual cockpit with constantly refreshing displays scanned via Active Camera and FSRecorder, it DOES happen but I don't complain.
That's the sort of simming I enjoy. In my view everything always comes for a price to pay. I am planning to buy a new powerful PC (with i7 hopefully) with Win7 64 bit.
I hope my FS9 experience will get a new power then. So far I am surprised how much my E8400 with 8800GT and the 32 bit WinXP can do for my simulation....