I'm pretty certain it's the Python side of things making it slow - I've been involved in a project which converted its PyQt interface to a native C++ Qt one, and the UI speed up in terms of responsiveness was significant.
It depends what the interface is doing - if it hasn't got deeply-nested layouts and widgets (like dropbox - it's just menus and simple dialogs), you can get away with it.
If you've got resizable windows with split nested views containing things like column lists or trees, you notice the overhead a lot more.