Upper limit for black hole’s rotational speed

[Eagle9]

From wikipedia:

Rotating black holes are formed in the gravitational collapse of a massive spinning star or from the collapse of a collection of stars or gas with a total non-zero angular momentum. As most stars rotate it is expected that most black holes in nature are rotating black holes. In late 2006, astronomers reported estimates of the spin rates of black holes in the Astrophysical Journal. A black hole in the Milky Way, GRS 1915+105, may rotate 1,150 times per second, approaching the theoretical upper limit.

Rotating black hole
So, the black holes have got the upper limit for rotating? Could somebody explain me why? I can understand one reason: due to special theory of relativity the event’s horizon’s circular/linear speed cannot be more than speed of light, right? Or maybe there is another reason for this upper limit?

 

[WannabeNewton]

See here: http://ttt.astro.su.se/groups/head/sara/node11.html i.e. there is a constraint on the spin if the event horizon is to actually exist.