Framerate killers!

I played around with my settings in game and I found that the game runs really smooth, frame rate a continues 60 (with seems to be the vsync option) on ultra textures and landscape. Settings for shading, building and trees are medium. No cumulus clouds in the sky. The game runs smooth as a babies buttocks.

Adding more trees to the scenery really makes the game start to stutter when these trees come into view.

Adding more the a few cumulus clouds really impact the fps also. Medium cumulus cloud density settings drop my frame rate to 40 something and make the game start to stutter quite badly.

I have a I5-3450 with 8 GB of gddr5
Game installed on a sdd
GPU Radeon R9 270 with 3GB of memory

Is it because the trees and clouds are not implemented the right way or is it because my system cant handle it?

My gpu and cpu dont seems to work very hard (40% - 50% of max capacity)

I found out that my gpu was not able to keep up with the ultra settings of the game. So I replaced my AMD radeon r9 270 for a geforce gtx 970. I have to test it for longer periods of time, but on ultra in a densely forested area it was more then fine! I hope for many enjoyable hours simming with my new gpu!