I think Adorno said he started getting a bit of lag after placing over a thousand objects (bricks in the walls of a broch) in one scene. The Star Wars team also had some lag problems on their maps with loads of vegetation. I don't know what effect elevation has, don't think anyone's ever tested that, though I did once fire a large passenger train very high into the air until it shot far out of my visibility (it's a long story) and there was no noticeable slowing, so objects can be safely placed very high up in the air.