I've started the work on improving the performance, my first step is to rework the vegetation and the way it's rendered. I'm still working on it but I already got impressive results.
I had one area in a special mission where frames droped from ~90 down to 55 frames per second (FPS) but for a good playable PC game you need at least 60 FPS so I reworked the Level of Detail (LOD) system of the vegetation.
The new Vegetation-LOD increased the FPS drastically! In the special area described above the FPS increased from 55 up to 85 which means a increased performance by 54%!
Even better: I'm actually rendering more vegetation with better performance as before.
In the screenshots you can see a the LOD system working in the mission Village (from the Alpha Demo). Each color represets a LOD stage and grey is the base mesh. You can see the LOD system working the best on the trees in the background. Those turquoise Trees are only 2D Shapes that are rendered very fast, if you would come closer to them, they will change to the trees you see in the foreground.