Any news on this?
On W8 it works, but FPS dwindles from 70-80 down to 20 when flying

On Windows 8.1 with latest invidia driver (as of 17/9/2018). with both methods , dxwnd and dxtweaker, I get a rounding down FPS, which goes from 60 down to 10.
Other than that ( ! ) it works fine
Any ideA?

Did you check the pinned threads? There's one talking about slowdowns and a couple more talking about graphical issues. I remember it usually being driver issues but I don't remember any definitive fixes.