A week ago for my Friday night session, my adventurers came across a pack of 18 Gnolls. 16 of them were your basic Gnoll fighter, one was a Gnoll Pack Leader, and the last was a Gnoll Fang of Yeenoghu (which was responsible for the rapid multiplying of the Gnoll Population).
I had planned that the group would meet these 18 Gnolls in 3 separate encounters, such that the group would only face six at a time. But the group decided to enter their area from 3 sides and, lo and behold, activated all 3 groups at once.
The Initiative order of 18 Gnolls plus my players was so extensive that one of my players fell asleep before his turn came around again (It was about 1am by that time). The next day me and one of the core players reflected a bit and it was brought to my attention that the encounter could have perhaps been handled differently.
What is the best way to I handle large creature counts?
While I am aware that I could create 3 enemies representing a group of Gnolls each, each a single mechanically strong enemy to ensure the encounter is just as hard, this does not give the right atmosphere if I want my players to feel overwhelmed by a multitude of lesser creatures. How do I give this overwhelming feeling, without an initiative round lasting for hours?