I never had these problems at Bitfinex.

If u wanted to place a Market Order now to buy BTC on Finex, you would get filled at or near spot, cos the market is reversing and there are probably real traders wanting to get out. If you placed a Mkt Order to sell BTC right now, just watch the Bid Orders vanish beneath you.....A few days ago, I put in a Short Sell mkt order with spot at $678.88...got filled at $675.25, and this was for 5 BTC.......it's all bullshit, and it's bad for you.
.....last night was particularly bad, because I was trying to catch a reversal. The market was moving moving fast, being lead as usual by China, with the Finex bots responding to action on OKCoin and Huobi, and front running normal Joe Finex traders. Joe may think there are a few dozen worth of bids in the Wall to take a bite out of, what he doesn't know is how many of them are genuine orders, and how many will evaporate the very minute he executes his market order.
You will encounter this slippage issue in everything you want to trade ... that's why advised trader always try to enter their trade in calm days, to avoid slippage cost (and also to avoid catching a train at midway to the last station)