Internet is slow on an iPad bought in New Zealand, but used in USA
EDGE is a technology that predates 3G. It is advertised as being capable of "peak bit rates" of 1 Mega*bit*, which is equivalent to about 128 Kilobytes per second, but in practice will be far less.

Anecdotally, cellular data reliability and speed in the Bay Area is rather miserable due to the vast amount of usage, and also the overall lack of towers within the bay area.

Even more amusingly in this scenario, iOS devices do not work with T-Mobile's 3G. Either the T-Mobile store rep was poorly educated, otherwise they were making a bold-faced lie to you, since they would be unable to provide a better solution, as they would only be able to provide EDGE capability to any iOS device as well.

According to this CNet article, Simple Mobile uses T-Mobile anyway:


  Enter Simple Mobile, an MVNO (Mobile Virtual Network Operator) that I confess I have not heard of until this month. This relatively young carrier alternative (it started business in 2009) offers even cheaper monthly rates, aimed primarily at those who have unlocked GSM phones. This MVNO runs on T-Mobile's network, meaning it uses T-Mobile's existing infrastructure for its service. Simple Mobile offers no phones of its own and relies on its customers to come up with their own hardware.


So using T-Mobile instead offers no better quality of service. Since you appear to have a GSM iPad, it would appear that your only chance of improved service would be using AT&amp;T, or a smaller carrier that leverages AT&amp;T's network.