Hammer, scanline is an accelerator that is used within MR its not the actual render itself, MR relys on Raytracing as the underlying render and will use this when it cant use scanline as scanline is limited to lenses, so its only used to speed up the render where it can, though as I say it will go to raytracing when it cant use scanline.

Having lots and lots of detail would ofcourse slow a render down but I highly doubt it would cause such high differences.