Central black hole spins at 84% of maximum possible speed*
How black hole spin fixes ultimate size
A black hole that is accreting mass must maintain its angular velocity. However, there is a limit to this process. Eventually, the black hole will reach a size such that the circumference is travelling at the speed of light, and this must set an upper limit on the maximum size that can be reached. This is what is known as a maximal black hole. With this new understanding, once a black hole reaches its maximal size, it can grow no more. More significantly, once a black hole is created, its' spin can only increase towards the maximum possible. As a consequence, the Universe will be predominately filled with black holes spinning at very high rotational speeds, a feature of all measurements of black hole spin made to date. It also follows that black holes formed from spinning giant stars we would expect to reach a maximum size of some forty times the mass of the sun.
This observation has such profound consequences, that it is worth framing it as a new law of black hole mechanics.