SFGUI will always try its best to squeeze the most performance out of even the most horrible code
.
But... as the others have already mentioned, why even bother with putting the controls
inside the SFML window in the first place?
Widget toolkits like Qt, wxWidgets and GTK are at least as portable as SFML, and are highly optimized and tailored for use cases like yours. It isn't that hard to embed an SFML drawing surface inside a control and draw to that instead. That way, you use SFML for what it is meant for: rendering. The rest of the complex UI that you will probably end up with in your editor will happily be taken care of by the widget toolkit.
There is currently no up-to-date tutorial on embedding SFML into other windows or controls, but if you resort to the example code, the API documentation and even the older (1.6) tutorials, you should be able to figure it out without too much effort. If you have questions, you can always ask them here.