GSX won't cause you any visible drop in performance.
If you are getting it only at night, it's possible the issue is the dynamic lights on the vehicles, which are added to the dynamic lights of the airplane and the eventual dynamic lights on the scenery. It's known that dynamic lights affect fps a lot depending on your antialiasing settings, try with different settings, which lowers the impact of DL.
If you are getting a noticeable fps loss at day too, that's unheard of. You should expect some fps loss, of course, nothing is "free", and if the sim is asked to display extra stuff, it will slow down a bit, but nothing more than adding a few AI more.
