Hello,
there are two behaviors side by side.
One is the lame behavior in the main menue. Fps drops to 10, 12 or 15 fps. This can be (often) solved by waiting some 15 seconds after the start of the sim without doing anything. Also there is some lame behavior after loading the flight in some areas when the sim loads the data. Here (often) helps to wait some seconds again or to switch to outside view, take an (stuttering) 360 degrees view and switch back to the cockpit. Not as intended but in bad times, every straw counts.
Second behavior is immediately drops of fps from somewhat around 30 in my case (G2 at full res and medium to high settings) down to 1 or 2 fps when crossing some invisible borders. This can happen everywhere and everytime, even after an 4 hour flight without any problem before.
Its two different things!