Why is better separation of two liquids achieved by slow rather than fast distillation?

because faster distillation usually means higher temperature. Under the circumstances, the mixture often evaporates too quickly for the two liquids to separate properly.

Also, distillation work on the principle of different boiling points for two liquids, if you distillate too quickly, its easy to overrun the first boiling point and get a bunch of unwanted impurities.