Not only for VR; it also happens in regular gameplay. For me, the framerate drops drastically later in the mission when they radio me to divert to Incirlik.
Hope ED o Ground can do something about it.
I also saw that tacview can be the problem, so I disable the module, even tho the framerate was better you can still feel a clear fps drop on the mission
P.S. Sorry for my English; I’m not a native speaker