I wonder if file size has anything to do with it. File size increases greatly with ISO. Not triple (so that is still odd), but it can double. I would think that anticipated file size would be part of the algorithm, but perhaps they only consider base ISO. I just checked on my 5DIII, and the number of pics projected to fill the buffer in the viewfinder does decrease with ISO (13 at base ISO, 12 at ISO 2000, and 10 at ISO 25,600), but maybe the 5DS(r) is different? That would be a bit ridiculous, but maybe. Also, at least with my 5DIII, it only goes down by 1 shot if I take 1 shot, so I am not seeing the same phenomena.

Then, the other question, can you rattle off 12 shots on a slow card (filling buffer) before it slows? Or is it 9?