Well, Poser is a REYES renderer, so it's splitting all the objects into pixel-sized micropolygons when rendering a shadow map. In addition, it supports displacements and transparencies in shadows, so it needs to evaluate the shader trees while creating the shadow maps. The 3ds max scanliner (I have no experience with Mental Ray, last time I touched 3ds max it was at version 4) is rendering its shadow maps with a very simple method, more like the Poser 4 scanline renderer or how Poser is rendering the preview in the document window. Also, don't forget that discreet is a much bigger company that can devote more human and monetary resources to speed optimizations. Of course, I would also like to have a choice here - it'd be cool if we could use a preview-style fast and dumb renderer for shadow maps and do the final pass on FireFly.