It is indeed tiled into 9 pieces, and I suspect the prepass is only using 50%, check the CPU usage when it is actually rendering the tile. It is good to have 9 tiles, because if you had say 6, it would mean that a tile on the dual core would take a lot more time processing, and wasting render time, the more tiles, the faster they render.
I wonder though why the prepass isn't using 100% power...