The problem with the big exchanges like Poloniex, Coinbase, Kraken is volume. As soon as they get a busy "bitcoin day", their systems get clogged and transfers get delayed. I've had withdrawals waiting a day before receiving the bloody email to confirm them. Not to forget the fact they get DDOS'd like a few weeks ago, which cost me dearly because I couldn't log on take some action during a big dip :(
One would consider using another exchange, but none of them offer most the altcoins that Poloniex does, so it's a trade-off.