you don't want to design a level with MED, it is for modeling
individual models.

you are correct, it takes a long time to build prefabs.
but the speed is actually faster with them in certain cases
(and you can lightmap them).

build time has nothing to do with the framerate of a game,
it is based on how you design your levels....
(gamestudio is very optimized, it can do a lot)....

also, there is a new way to make levels, it is called "GED",
I was waiting for months for that, and am very happy!

-Jim