I haven't checked the numbers in any way, but is it safe to assume that the black hole is not rotating "very fast"? In the news covering the render software and papers that will be/have been published, it was mentioned that they were surprised to see the effect the rotation of the black hole has on the accretion disk -- that it bends and twists space and light, so insted of a flat, Saturn-ring-lik disk, you end up with the thing you see in screen.