So my Global Illumination Voxel Engine Research is making progress. I've added colour, indirect lighting, and also SIMD intersection tests. The SIMD is AVX 8-wide. And the speed up compared to scalar code is 4.5x which leads me to believe there is some room left in optimizing it. It now runs 38fps on a single 3.2GHz Haswell core.
Notice the low-frequency noise. Photon Mappers typically have high frequency noise, and path tracers that update only some of the screen's pixels have even higher frequency noise. The noise in my renderer is much more pleasant, and reminds me more of oscillating electrical lighting, or maybe the flickering of a fireplace flame.