I notice a decrease in performance with more features on a planet. I also notice on a flat planet with only one small pond there is a great increase in performance in comparison to the planet with many features including but not limited to mountains and cracks.
More things = More work for your computer to do . The flat planet bug is known (See http://pa.lennardf1989.com/Tracker/inde ... ask_id=981)
In the final game, you can probably choose feature density during planet creation. But this can't be changed per-person, because trees and other objects will presumably affect the game at some point (collisions etc.) For now, just try turning your graphics settings down if they aren't already, and running in a smaller window.
If trees remain other wise useless and just for looks most likely it wil be a setting but currently no built in way. Might be worth your time to attempt to mess with the files and delete a few thing just for the hell of it. And if that helps might as well share your results. other people have had this same problem.
