The I3 Connectivity Explorer works hard to keep data handy, but even with a fast internet connection, you’re sometimes downloading lots of map data (think about looking at all the Census tracts in Michigan or California). Also, a couple of the Census queries are greedy about pulling down data you might want to look at, so those queries run slow. Having Multiple Localities in a notebook can also increase the data fetch times.
Finally, if you’re working on your own broadband issues, there’s a good chance that your own connection, local network, or computer are slow. It’s been an “interesting” learning curve to try to provide lots of data about Internet connectivity over slow or unstable Internet connections.