This is why when you see arbitrage opportunities, buy the tokens from the cheaper site and hold. The price difference from the two exchanges will automatically even out leaving you with profit without having to worry about transfer times.
This is how arbitrage is done correctly.
This is not always the case. Sometimes the price can even out at a lower price than the one you bought at, even if it was cheapest price at that time.
If you are quick enough, have good internet connection, know how to set up gas and have a good amount of ether to play with you can do multiple buys and sells and gain big profits.