1. Make sure you are using the opengl engine in the 3d window.
2. Turn off texures in opengl options if necessary
3. Use a marquee to restrict the limit of what is generated in 3d if necessary
Most Archicad users neurotically go about replicating contour lines that result in too many nodes/polygons. It is better to sample heights yourself, adding more nodes where it is crucial, using fewer when not.