Hey justicetea,
This is a major part of any open world game or project that is using a very large landscape with foliage and other meshes etc.
Now, without looking at your scene I can tell you that creating multiple LOD’s for your foliage and other meshes change at the correct distances, as well as using the built in Landscape LOD feature, will help increase your framerates and boost performance.
Essentially what this is doing is reducing the number of triangles being drawn at further distances, since that detail is not needed and won’t be visible. It’s the same for both Landscapes and regular static or skeletal meshes. Keeping your high poly stuff close to the user to increase the amount of detail that is seen, and making sure you are not wasting triangles and vertices off in the distance is an important stage of optimization for performance.
Let me know if you have further questions.
Thanks,