Black holes spin almost at the speed of light
forbes.com
forbes.com
Almost every atom entering the event horizon goes in tangentially, because the target is so small, falling from effectively infinite distance, which means it should be going only infinitesimally less than lightspeed at that moment.
The only consumers of kinetic energy would be other particles that happen to enter orbit at a different angle, and collide; and electromagnetic interactions, producing e.g. AGN galactic jets. How much of the kinetic energy gets diverted into those?
I see assertions that the singularity is toroidal, so the actual center of the black hole is not itself singular. Can we posit a fully toroidal event horizon, with a hole through it? Might we expect almost all black holes to be that way?
It's not hard to imagine a black hole spinning one way to get an accretion disk going the opposite way, so I would expect to find the occasional not-spinning-much hole.
But this appears to be more complicated:
https://physics.stackexchange.com/questions/183832/typical-r...
https://physics.stackexchange.com/questions/174247/is-there-...
Taken from one of the stackexchange posts, this is quite interesting: http://www.nature.com/news/spin-rate-of-black-holes-pinned-d...
The angular momentum inside the black hole should then also reflect this, but I don't know if physicists are too confident about what the matter inside a black hole looks like.
Theoretically a black hole can spin faster than the speed of light since the event horizon is just space, not matter, and space can certainly exceed the speed of light (see expansion of the universe).
IIRC from relevant literature, the event horizon shrinks with increasing speed until it vanishes once the speed of light is exceeded and leaves behind a naked singularity; a black hole with no event horizon.
A black hole without an event horizon is a naked singularity[1], a concept in theoretical physics resulting from looking at the mathematical equations that describe black holes and observing that the event horizon's radius becomes undefined at certain angular velocities[2]. It's not clear if it can exist in reality.
AFAIK (not a physicist), gravitational force of a specific object (as a function of distance) cannot be discontinuous, can it? It's definitely not under the classical F=G(M1)(M2)/r^2 equation. Force can't be discontinuous unless mass changes discontinuously or space changes discontinuously. But I don't know if there are extreme conditions (and a naked singularity might qualify) where that equation breaks down, or is thought to break down?
[0] to support the movie Interstellar, several people worked on an accurate rendering, based on known physics, of a rotating black hole: https://astronomynow.com/2015/02/14/interstellar-technology-...
[1] https://en.wikipedia.org/wiki/Naked_singularity
[2] https://physics.stackexchange.com/questions/147034/rotating-...
I think it's often assumed that most black holes in the universe would have relatively low spin, compared to their mass. The research here is saying that, perhaps, the way that stars collapse would produce near-extremal holes in most cases, so they would be common in the universe.
I recommend reading up on black hole geometry ad Kerr(-Newman) holes; it's likely that any description that fits in a HN comment will leave you knowing less than when you started.