I always thought it's some kind of artificially generated headroom by DCS, so your GPU is not running at 100% performance, but has extra capacity to handle something like a bunch of explosions. The problem is that, this is just not enough for VR, where the GPU has to serve 2 displays. They optimized something about this recently, but high settings and 90fps are still not possible (for most situations).
So the 2 options are: high fps at lower settings or higher settings with reprojection (or AWS).
I just decided to go for higher settings and quickly got used to the 45 fps.
Btw, I tried many settings and I don't think MSAA and shadows are the most demanding settings. I found high visible range to be much more problematic!