I noticed there are diminishing returns when rendering. My fastest time was rendering in a single tile for the full image at roughly 1:17:17 (minutes/seconds/miliseconds), while doing it in 400x400 tiles was slower only by a few miliseconds at about 1:17:24.
Same goes for CPU rendering, which saw diminishing returns as the tiles got too small, going from roughly 3:30:21 at 60x60 tiles, to 3:29:97 with 8x8 tiles.
(My GPU is a gtx 970 turbo, and my CPU a 4.0 GHz intel i7 4790k with hyperthreading)