be very careful when using one of these big exchanges such as poloniex for day trading and having short positions which you would like to liquidate as fast as possible by executing an order on the spot. because they go down or become drastically slow in times of high activity and you can easily lose the opportunity to sell because their servers can not handle high traffic!
BTC-E experiences the exact same down and slow periods. It has been like that from the very beginning of me using that exchange. Large players know this, and exploit this weak spot by DDoS attacking these exchanges briefly. I don't really know why they can't just keep running smoothly since other major exchanges can do it. It's not a money thing since both BTC-E and Poloniex have massive fee incomes.