i just thought that as it works on a row by row basis, it decides on a box size and goes along rendering. if it comes to an area of high detail then it will stop if the box is too big and make a new smaller box and then go again.
If the scene has lots of areas of varying detail it would be doing this lots and maybe this would be the reason for long render times? I might be wrong i'm not that technical, but i do like numbers.