It's important to note the difference between slow internet and high latency.

The speed of your internet is the maximum amount of data that can be downloaded in one second under optimal conditions. Under optimal conditions, a 6mbps connection will be able to transfer 6 megabits of information each second, for example. In an MMO, slow internet speed can typically lead to things taking a long time to load but generally they would have to be extremely slow to cause the game to become laggy and unresponsive.

The latency is the time that it takes for one packet of information to travel between your computer and the remote server. High latency can lead to the server taking a long time to respond to your keypresses, attacks hit you even though you moved out of range in time, you'll get spikes of lag where everything will stop moving and then "fast forward" into place after a few seconds.

Latency is the real killer when it comes to MMOs, not slow speed so you're best off enquiring to your ISP if you're worried about that. 6mbps is more than enough to play this game.