Ok, the test I did was on a 8km x 8km landscape (100 tiles). Spawning in foliage over this entire area dropped my framerate to 18, which is what I would expect in that circumstance as I had no culling or proper LODs set up to prevent a framerate drop. Have you set up occlusion to prevent rendering outside of a specified distance so that you can save on resources? Adjusting this may help with what you are trying to accomplish as well.