Anything using WED blocks will cause a radical slow down, even map entities that contain any block. Of course, a map entity could contain MDL models. HMP and MDL files have the very fastest of rendering speeds. Plus, they allow for much more flexibility in shape, design, structure, and they are dozens of times faster at rendering. MDL terrains are better than HMP terrains in that they support far more flexibility in shape, sometimes even a faster frame rate (depending on the design). MDL rocks!