Pointclouds and brickmaps can improve the render performance greatly since they “bake” a 3d model with any shader data that is needed for the render just once, and can be used over and over again. In some cases the original model doesn’t even have to be used for the render, brickmap which is free...
This page shows the results of applying a custom surface shader, written in the RenderMan Shading Language, to a Maya/Houdini scene in order to create the illusion of decay and rotting.