I was reading "the science behind interstellar", and found that Kip Thorne purposefully used a SMBH in his simulation which had its maximum attainable spin around its own axis. He argued that this was needed to be done in order to achieve the extreme time-dilation depicted in the movie (an hour on Miller's planet was 7 years of Earth time). I'm really interested to understand the theory behind this. Particularly, why does a rotating black-hole have a stronger gravitational pull than a black-hole with the same mass but no spin.
I think it has to do with its increase in rotational kinetic energy contributing to a further increase in its mass, but I'm not sure about this.