I was just playing around with the light portals and rendering settings and realised that in 2.78 (or at least recently) they added the render order “Hilbert Spiral”.
You can have a look at some of the commit message here.
If you are looking to save rendering time, it is worth considering the “Bottom to Top” tile order. I just did a very simple light portal render and for reference:
- “Center” order - 2m27s
- “Bottom to Top” order - 2m02s
- “Hilbert Spiral” order - 2m12s
So if you are not using environmental lighting, or simply don’t get much of a performance benefit from using portals, try using a different tile rendering order like Bottom to Top. Center or Hilbert Spiral might be good active development where you want to see the effect on the focal point of your artwork early in the rendering process. But when you are doing a final render or know you are going to render the whole scene, Bottom to Top is faster.