Terrain types...

That is actually pretty much as fast as its going to get because quad trees are fast. There are fancier ways of doing it like graphs and or using an area of an array, accessing it by using a position matrix. This almost makes hidden surface removal for (out of possible range of view) not nessessary, but you can possibly find a way to incorporate this in to the method you are already using. If I am not explaining this very well, don''t worry. Your method is actually good enough in my opinion. Since graphics is what normally slows programs down the most, trying to prevent polygons from being going through rasterization and some other rendering steps is most crutial.