Black holes are cosmic spinning tops. Their spin warps spacetime (as predicted by Einstein) and is described by a spin parameter from 0 to 1. The limit 1 is unattainable: infalling matter acts like a brake. Back in 1916, Schwarzschild found a solution for a non-rotating hole, and later Kerr — for a rotating one.
Comparing two nearly maximal spin models (0.9375 and 0.998), scientists saw that their photometric images and the behavior of the hot disk are indistinguishable. The photon ring — the boundary where light goes in circles — ceases to change at such speeds. Even the Event Horizon Telescope can't spot the difference.
Surprise: the photon ring's thickness is ten thousand times smaller than the black hole itself. If the hole were as big as Earth, the ring would be as thin as a hair. Only a future space telescope aimed at measuring this boundary will be able to discern the differences.
The fastest known black hole spins at 0.99 of the limit — spun up by an infalling disk, like a whip spins a top.
In the film Interstellar, light is realistically shown bending around the rapidly spinning black hole Gargantua.