Thanks for your hard work @Proot!
I gave the prerelease a try. Most of the stuff looks great! The parts, the planets, etc.
The problem seems to be the clouds. Kerbin's cloud layers seem to behave oddly. When viewed from orbit there's a cloud layer that's dark and shadowed even on the day side and even with no other cloud layer present above/under it. The volumetric clouds seemed completely broken for me. They looked like huge squares which cast a wavy glimmering shadow on the terrain on the day side, turned everything black and murky on the night side and gave a huge performance hit. In short - There's something wrong with them which causes them to look odd and cause scatterer to cast bad shadows, which I suspect is a cause for the performance hit.