"How can there be many different versions of this issue"
Seems to be how this game is, everyone is on different pc, settings and equipment, what works for me, might not work for you. I get roughly 25fps flying A310, not too bad for me personally but if I ALT-GRD and pop out the PFD or ND I get average of 55fps. I do not know why, I thought it would be opposite and lower FPS, guess you just have to experiment for your own systems