Well that is the problem for certain. When you have a large landscape with a lot of foliage being rendered all at once it will cause lag no matter what your system specs are. What you will want to do is use cull distance to only render the foliage you can see at that moment, which will reduce your draw calls immensely. I would also recommend level streaming for different areas of your map to prevent rendering a lot of unnecessary portions of the map at once. You can use the “move to level” landscape feature to move individual tiles of landscape to sublevels in the levels window to help you determine how you want streaming to occur.