C# is a compiled language at all levels (source into bytecode, then bytecode into machine code either JIT or AOT). V8 has JIT compilation for hot paths. As a result, JS is significantly faster than the interpreted languages like Python, Ruby and Erlang/Elixir/Gleam.
No one under GTK/Gnome uses plain C, they use Glib as a wrapper. Plain ANSI C might be 'unusable' for modern UI needs, but, as I said, just have a look on WebkitGTK4.
Glib everywhere, WebkitSettings are a breeze to setup.
Vala it's a toy because Miguel de Icaza went full MS with C# since Ximian. If Vala had more support from Red Hat, Gnome 4 could support Vala as the main language for it. JS? Lua and Luajit wouldb be a better choice for Mutter scripting. If you have a look on how Luakit and Vimb behave, the difference it's almost nil.
