Build time has to do mainly with the number of blocks you use. If it takes 4 minutes to build a level with 1200 blocks, it'd take 16 minutes to build a level with 2400 blocks. I'm just saying that level geometry renders much more slowly than models. Try creating a mess of about 600 cube-shaped blocks. Then create a single model that's exactly identical to this and you should recognize if there's a difference. Using 600 models is not the same though. Level geometry can impact the frame rate, it just has a stronger influence on it than regular models do.