Many of the withdrawals from Poloniex were almost instantaneous.
That leaves me wondering how do they initiate something that comes from users input ("hot side") to send out coins from the "cold" side so quickly?
They have their cold storage ready as if you see they send many big transactions daily, usually every hour they send 1 mln dollars in value, bitcoin transaction which includes all withdrawals from the users that's why their withdrawals come fast but not as fast as you are describing, sometimes I have had to wait for more than 30 minutes before I got my withdrawal.