Then there has to be something wrong with the building.
Mesh not closed properly, shader or material effect, shadow.

That's what I'd be checking anyways. Remove the building and try the level again, If the framerate is up you know it has to be the problem.

Windows XP SP3
Intel Dual Core CPU: E5200 @ 2.5GHz
4.00GB DDR3 Ram
ASUS P5G41T-M LX
PCIE x16 GeForce GTS 450 1Gb
SB Audigy 4
Spyware Doctor with AntiVirus