A Center for Astrophysics-led team has measured a black hole spinning so rapidly that it pushes the predicted speed limit for black hole rotation. It is turning at more than 57 thousand RPM, according to a SpaceDaily article.
The maximum RPM is determined by the mass of the black hole, calculated using the Kerr limit. The Kerr metric is an exact solution to Einstein's field equations and spinning black holes are also known as Kerr black holes.

Artwork Credit: David A. Aguilar (CfA). It depicts an accretion disc around a Kerr black hole. The white band near the center is where the matter to be eaten by the black hole emits vast amounts of energy in the X-ray band.