except c-cex and sometimes yobit, all of the exchanges that i have seen have been pretty fast. both their website (the web interface you visit) and also their API. and that is most of the times not always.
every single one of these exchanges without an exception become very slow and even go down during high traffic times for example when there is a big dump going on and everyone is dumping their coins.
I think he is talking about those exchanges where there is a huge amount of people trading where he can easily do trade with highest amount of traders. So in this case I will suggest him to do trade on binance, bittrex and poloniex or cryptopia. These are suggested by some of my friends also and they are also happy with their trade on there. The trade volume on binance and bittrex is much more than the remaining two.
I also think that op is talking about the number of traders and about the volume. If this is the case then binance will be the best choice for alt now and the second choice will be polo, cryptopia and bittrex.
There is another good exchange which is getting famous now is Kucoin and they are also running some good amount of promotion as like binance.
And for USD trading bitfinex can be the best choice due to its volume.
