That's not much good when you're just talking to ahitty GObject APIs that - guess what! - use reference counted GC and have lots of race conditions. You gain nothing
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++.