I've written before about various optimizations I've done to Spanning Feed Builder, but I just made a change that will have a dramatic impact, not only on load times of the S-Control, but also on my bandwidth bill.
The only downside is that in the rare event a user modifies his object model, say, by adding a new custom application, object, or field, the Feeds tab will have out-of-date information—but only for the duration of the current session. So logging out and logging back in brings the Feeds tab up to date. (Navigating around the site long enough for the session ID to change will work too.)
So one big optimization is done, and only one design flaw is yet to be addressed before next week's announcement.
1 Unless that browser is Mac OS X Safari, which completely ignores cache directives. Ugh.