Long term Poloniex user here, id definately recommend their site to folk who havnt done a great deal of trading before. Its easy to use with alot of helpful hints dotted around the site.
There are occasions during fast price movements where it can become a little laggy but this is down to the sheer volume of folk using it. The recently removed the troll box (chat box) which in my opinion is a good thing, it caused epilepsy trying to keep up with what was being said, the majority of the chat being folk trying to pump obscure coins up to reap rewards. Now you're alone with a chart without any of the noise from the crowd... which is a far better way of trading.
RE: Top Trading Platforms for STEEM...