Ok, as I said it was either that or poor coding. Also I was fiddling around with nuke in the FTB mod and there is huge lag when it exploid and after with all the bricks on the ground, I take it this is also due to very poor coding.
In that case, as is with any game, a huge mass of items on the ground will always cause lag. Good systems have a higher upper limit, of course, but even the best hardware can only handle so much.