<< just grass bricks and trees. >> Imported grass can be a rather big file sometime, if it's been duplicated many times it can cause slow downs. Trees are the same, are you talking a wall of imported individual bricks? that could be large to. Here's a couple of thing that may help speed things up some. Work in solo mode when ever possible. Change the wireframe resolution lower ( static, selected and motion ) turn the nano preview to wireframe mode.

Stephen Ray