Assume a spherical black hole that is eating matter from its surroundings. Then its Schwarzschild radius will increase with a speed proportional to the mass flux that enters the black hole. The question is: is this speed limited by the speed of light in vacuum c? If the event horizon is a purely geometrical notion, then I'd say it can grow at any speed. But if the event horizon were holding some quantum fluctuations with attached energy, which move with the event horizon, then the horizon would be a physical object with mass and its speed would be limited by c. It would also be interesting whether a speed of growth larger than c could be observed, i.e. if it could exist relative to a stationary or to a falling referential. Btw, in order to obtain a speed of growth equal to c, a rough classical estimate gives a mass flux of about 100 000 Suns / second. Not unachievable for a supermassive black hole, yet this is far higher than the (average) mass flux of a quasar.