A more complicated scene this time. 505 vertices and 901 faces, with reflective surfaces (4 bounce limit). In fact, this would be a good opportunity for u-v calculated materials: the checkerboard floor in this scene is made of white and black triangles, while a "shader" to do the same would need only two triangles, setting the color based on uv position.
More primitives would help too. The icospheres are 20-sided, but a real sphere primitive would be much more accurate.
This 640x640 image comes from the PC, and took 263 seconds to complete.
The Arduino worked overnight on the same scene and didn't complete - at the same rate as before, it'd need to run for 4.26 days : ) A 64x64 version is rendering now.