My gut feeling is that in SU3 the framerate decreases with the duration of your session and the number of menus and windows that are opened and closed. So, if I don’t need them at that moment, I keep all windows closed rather than minimised, and I also try to minimise fiddling around in settings and EFB during flight.
This behaviour might be related to a bug which was already confirmed by Asobo:
Apart from that I think SU3 generally deceases fps compared to SU2 (it’s acceptable for me tho), but performance improvements will be the main focus now for SU4. So let’s be hopeful. I’ll jump the SU4 beta gun the moment it starts. 