Yeh, thats the big reason I don't like to do assembly in postwork.  Getting the lighting to look normal is a real pain.  The point is though, that when you pay as much as people do for a program for 3D modeling and rendering. you expect the software to be able to handle more than a very simple dumbed down scene.  My machine is no slouch.  I built it specifically for 3D modeling, It has a Phenom 9600 Quad Core CPU, 4GB of fast corsair RAM, An MSI motherboard, and a really good ATI 3450 video card.  It also has 3 really fast Seagate 1TB drives.  If this configuration can't handle a fairly complex scene, then there is something wrong with Carrara.