Best: using CPU with 12x12 tiles: 1:50.99
Changing the tile layout to Hilbert Spiral from Center, I cut the time down to 1:48.61
My best GPU time was 2:06.02 with 400x400 tiles
Worst: GPU with 25x25 tiles… over 20 minutes
I also played with the open subdiv options, but observed no significant differences using it.
(and 8GB of RAM for those interested.)