I agree this is a very good thing by itself, but let's be clear that the extremly poor programmer ergonomics of the GTK-via-C stack have been superseded by nicer kit long ago (even within the GTK community).
This is how Mac apps tend to be fast: the high-level application logic is reference-counted and uses relatively slow Objective-C dynamic method calls, but things that are performance critical (CoreGraphics, VideoToolbox/AudioCodec, OpenGL, sqlite, the Mach kernel) are written in C and C++.