This week I have been adding LOD (which is pretty hard for terrains, because cracks appear between the high resolution chunks and the low resolution ones.) Easier was adding frustum clipping. Actually I simply clip against the left/right/top/bottom planes, and don't bother with near/far planes.
I also did some profiling and optimization. My old Mac Book Air with HD3000 graphics is struggling. Both the GPU and the CPU are a bit too slow, especially when a lot of dirt particles are created.