One place we should definitely not skip when trying to improve our render time is the Performance panel in Render tab.
Rendering to tiles makes better use of the processor power. In the newest Blender version, the numbers in the tiles section means their size in pixels, but understanding which size works best is a bit baffling. Tests showed that GPUs work best with 256 x 256 tiles while CPU makes best use of the 64 x 64 tile size (regardless of whether we are talking about BI or Cycles on CPU).