You know that all the Wayland primitives, event handling and drawing in gnome-shell are handled in C/native code through Mutter, right ? The JavaScript in gnome-shell is the cherry on top for scripting, similar to C#/Lua (or any GCed language) in game engines, elisp in Emacs, event JS in QtQuick/QML.

It is not the performance bottleneck people seem to believe.