Well, speed (and latency) is of course dependant on many many things, including the method and quality of the connection used to connect the phone to the computer, the quality of the cell phone itself, the quality of the provider and its towers (cell signal strength/quality), etc, all those (and more) factors can produce errors which in turn produce retransmission of packets, which in turn reduces usable bandwidth (which during the initial handshake will be reflected by a reduced "Connected at" speed, but it's possible to have it connect at a good speed, then still have enough errors to reduce the bandwidth (or worse, get booted) during the session), also some ISPs' modem pools won't let you connect if the initial speed isn't at a certain level of quality.
Considering there are many people using these methods, it seems to me at least, that they probably feel the speed is satisfactory, of course that too differs from user to user, just getting email?, it should be fine, want to stream realaudio?...probably not going to have a good time.