That's still JS. React Native doesn't get rid of JS, it just exposes native platform APIs to JS.
JS isn't the problem though. V8 is pretty efficient and lightweight. The problem is that Electron pulls in all of the DOM stuff when there are lighter alternatives built into most operating systems.
> V8 is pretty efficient and lightweight
How much lightweight? Similar to Lua?
Yeah, but not with the full weight of node, js or (html)DOM.
For the UI something like TCL or Lua or just some DSL is better.