Supermassive black holes: How did they grow so fast?

Fish Finders? Go:

Whilst astronomers are fairly certain that supermassive black holes, billions of times more massive than our  Sun, dwell at the center of almost every galaxy in the Universe, they are still unsure how these cosmic monsters reach such tremendous sizes.

The problem seems to be one of time, the Universe is 14 billion years old, and recent observations seem to confirm that such black holes were already present when it was just 800 million years old — thus in its relative infancy. How could these black holes have accrued so much matter to supermassive status in such a, relatively, short space of time?