I'd actually prefer a desktop app that does some of these things, considering I don't want to install Air for Balsamiq, and don't really want to do all my work in a browser, on a service that could disappear at any moment.
Sadly, I'm not the person with the expertise to do it.
Non-native, true. But slow? Balsamiq on my PC is nothing but snappy. You should consider sending a support request in case there is a bug in Balsamiq(or Air?) with your OS/pc configuration.
Sadly, I'm not the person with the expertise to do it.