A relativistically spinning supermassive black hole might not be very unrealistic indeed, since it will have ingested the angular momentum of a whole galaxy during its growth and concentrated it in a very small region of space.
> I found that Chris’s huge slowing of time requires Gargantua to spin almost as fast as the maximum: less than the maximum by about one part in 100 trillion. In most of my science interpretations of Interstellar, I assume this spin.
> In 1975, I discovered a mechanism by which Nature protects black holes from spinning faster than the maximum: When it gets close to the maximum spin, a black hole has difficulty capturing objects that orbit in the same direction as the hole rotates and that therefore, when captured, increase the hole’s spin. But the hole easily captures things that orbit opposite to its spin and that, when captured, slow the hole’s spin. Therefore, the spin is easily slowed, when it gets close to the maximum.
> Ultimately, when the hole’s spin reaches 0.998 of the maximum, an equilibrium is reached, with spin-down by the captured photons precisely counteracting spin-up by the accreting gas. This equilibrium appears to be somewhat robust.
In essence, the necessary spin (for an object to stably orbit at a distance for the time dilation needed for the plot), while possible, is not sustainable, based on what we know about physics. But it's allowable as real physics, because there could be some exotic physics involved in keeping Gargantua spinning that fast, possibly intervention by the civilization that created the tesseract.
