I don't think a "public" computer would work because you also need a little bit of software to be loaded on to the computer before starting. That is usually blocked because you are not an "Admin" login. If you have your own laptop, you could go to a Starbucks, McD, BK or something else and hang out for a while (once a year if all goes well). All of the ones I go in lately have free internet connections that are pretty fast. It took my 765T about 2.5 hours last week to update the latest maps and software with a wireless cable connection at home.
As for the roads disappearing, it could be a setting on the GPS. Make sure you have the road or street detail set to highest if you are zooming out. It may slow the device down a little bit when panning though. Other than that, you are at the mercy of the device. That is why I carry a paper map in an unfamiliar state (they are still free at the welcome centers if you ask). At least I can sit there and study it if I need to. I even carry my own state map with me. Of course I worked for a map company for 25 years ...