What is the difference?
Real Assault---Level of detail, # of brushes, the fact that it is all one big zone and the occlusion time this causes, all the scripted stuff ( mortars), method of construction, number of polys, poly/node ratio, # of sounds, etc.
Manor Farm--Level of detail, # of brushes, # of polys especially when you are out front looking at a lot of the buildings, occlusion time, # of movers ( doors and breaking windows), # of sounds, etc.
All these things will effect the amount of FPS you get. Plus the settings you have your game on, high detail actors etc.
This still does'nt make these bad maps, in fact they are a couple of the nicest ones out there.