Most internet cafes run at least two networks. I personally do all the networking and computer repair for a local business in that field. I setup a public and private access that each has their own internet feed. There is no loss in performance with this setup. The business back-end does not need a powerful connection, even a roadrunner lite package would work for that. If the business is just starting you can run one connection and have the public use the 2.4GHz stream and then use a 5GHz stream for the business computers.

Connection speed will always suffer the more people there are no matter what kind of a pipe line you offer. Even a T1 or T3 will get bogged down with enough people on it, however you can setup each terminal or connection to have limited speed based on what your pipe will support, that way you ensure each customer receives the same speed without a loss.