I read in the manual somewhere (forgot where) that one should only use block geometry to build the basics of your level elements only and then use models and entities to fill in the details.
Well, Ive been trialling this in the last day or so and it does work well then oddly enough, I went back to modelling with blocks again after a brief period of pain some years ago (too impatient then)and its all very fast if you use 'snap' and break up the level into definable blocks (rooms with passages leading to next room type of thing), no extra 'jutting' out peices, set flag none to surfaces the camera wont see to tell the camera not to look there, so speeding up buiiild time.
Models are fast but you need to "import" them to make them static or write my.dynamic = off;so that you can shadow map them or stuff like that