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.
At first when I was a newbie I was really got confused between the two exchange and I really thought that they are just the same.Bittrex is a good alternative exchange when you don't like to trade up with poloniex. For now I don't have any trust to make trades on bitfinex since they got being hacked in the past. I lost my trust and confidence on trading the site. C-cex is also a good alternative.