From what I've heard bittrex seems like the only reliable option, but some users are concerned about their site security, because of a hack last year, so I don't know if it is a good option or not. For short time trading the lag is an issue, but if you try to make long term trading then it should be ok to continue using Poloniex.
You're confusing Bittrex with Bitfinex. Bittrex was not hacked and they know how to properly scale their infrastructure to handle increased loads. They will also be rolling out a new interface from what I hear, but their main priority is meeting demand and security.