can the crystalspace engine draw a world with 100.000 - 1.000.000 polygons? Is the engine optimized to render a lot of polygons in one pass? There is a plugin called thing.dll how manage the rendering. In thing.cpp i could see, that each polygon is touched once on every render pass. This is not good. Are there any other plugins, that sort all Polygons by Material and render them all at once?
First this will depend heavily on your hardware. Where did you see that every polygon is touched once on every render pass? I strongly doubt that is happening. Can you show us where you saw that?
Genmesh is the best mesh object to use in general.