Here are the trial render times of the composite image Final Render shown below, all done in Cycles renderer in Blender 2.68a :
First, the carpet render layer, using CPU / experimental set using my lousy i3 processor 😦
A strange phenomenon here is that square tiles seem to take longer than rectilinear ones, the fastest being the 128×72 px tile size, but I guess it would vary with different CPU. Seems like emission number doesn’t affect rendering times (much)? Generally, the rendering times are not very different, from 1hr22min to 1hr47min.
But when it comes to the GPU processing, the difference is quite marked …
The smaller the tile, the worse the GPU performed, from a unbelievable 2hr32min using 48×48 to 47min using the image as 1 gigantic tile of 1920×1080. Phew! Render sample at 800, bounce 4.
Wonder what happens if tile size is set bigger than the resulting image … hmmmm?
Really have to improve on the carpet, looks way too thin 😕