Hey,
I am sorry for not replying earlier, I was quite busy figureing out where I messed up. Afterall I´ve decided to start new. I tried out alot of things, even went so far to wipe it clean to see if my landscape would at least run lag free when cooked. Well It didn´t.
You can only learn, its trial and error. In my case more error then trial but nonetheless I wont give up! The landscape on my new map is already done thx to Scape. A nice free tool to create heightmaps.
The trees are growing nicely and I am running at a constant of 40 to 55 FPS. Even in high density areas. In the next days I´ll give it another try and cook my map. Fingers crossed!