If anyone else is also experiencing the framerate issue when pulling positive Gs triggers the vapour effects: I found this workaround in another thread:
Of course this gets rid of the cool vapour effects (those of other aircraft like the F/A-18 as well), but it’s a small price to pay for a smooth flying experience in the F-35, especially when you can only see the vapours from the cockpit, when you push your head through the canopy glass. 