Hmm, I'm all for keeping load times as low as possible, but one of the main advantages of Mailman over Google Groups for example is its ability to be deployed privately, for example behind a firewall, on an intranet. Some companies keep a close eye on outgoing internet connections (with authenticating proxies for example, blacklists, even whitelists sometimes), some even only allow internet access from different physical hardware. I think HyperKitty should work on those environments too, since it's just a question of keeping a local version of the bootstrap CSS file.