Hi,
I don’t think you can a lot about that, other than removing unused widgets.
mxui.js is minimzed, or at least consists of a collection of minimized scripts.
widgets.js is a different topic – I read something about that quite a while ago: I think there were some problems in bundling and minifying the widgets without breaking stuff.
Would be interesting what Mendix has to say on that.
regards, Fabian