I asked Tor Lillqvist about this emulation thingy, here's what he said (used with permission):

"Who would be emulating what? Especially now with cairo, most of the low-level code has been designed cross-platform from the start.

Initially indeed much work in the GTK+ porting effort was caused by the need to "emulate" low-level X11 behaviour, as GDK at first was mostly a thin layer on top of Xlib.

A GTK+ application that behaves pathologically might be much slower on Windows than on X11, because some of the GDK API is trivially implemented on top of X11, but requires lots of work on top of GDI.

GTK+ isn't any more "emulated" than the "native" widgets (the Microsoft "Common Controls", I think they are called) are. Both are built on top of the Win32 GDI.

As to wxWidgets, it uses GTK+ on X11 (Unix), but not on Windows. On Windows it presumably uses the Common Controls.

Well, there certainly are bugs and misfeatures in GTK+ on Windows, for instance related to the concepts of transient windows, window groups etc. This can be irritating indeed in applications that use lots of top-level windows, like GIMP (and presumably Inkscape, I haven't used that). I wouldn't say that "treated like an application" is correct, though. These problems will be fixed eventually.

--tml"

Pretty much says it all, doesn't it