I am curious as to how rapidly most black holes are spinning. A refinement of the question could be: how rapidly are typical supermassive black holes spinning?
I ask because I have the impression that we have not measured the spins of too many black holes, and of the ones we have measured, a significant $\mathcal{O}(1)$ fraction have been spinning at essentially the speed of light, which is to say the angular momentum parameter in the associated Kerr metric would be very close to its extremal bound, $a/M \simeq 1$. For example, a very famous measurement found that the black hole in a binary black hole-star system was rotating at $a/M \geq 0.98$, http://arxiv.org/abs/astro-ph/0606076. I think this measurement surprised people, but I'm wondering if it has now become the rule, rather than the exception. Interestingly, it's much harder to measure the mass than it is $a/M$.
One reason why this is an interesting question to ask, because it concerns possibly the most abundant extractable energy source in the universe. If all black holes are rotating close to extremality, then we or some alien civilation could in principle extract $\sim 29\%$ of their mass-energy via the Penrose-Process. For a solar mass black hole this amounts to far more energy than the Sun outputs in it's lifetime.
And let me self-edit and point out a related post, Are "typical" black holes rotating or static?, but I am asking a slightly different question here. Regarding this previous post, of course typical black holes are rotating (and stationary, as opposed to non-rotating and static), and in fact all physical black holes are rotating to some extent because it is an un-physical idealization to engineer a perfectly symmetric and static collapse scenario that could produce a perfect Schwarzschild exterior.
