It would probably be about the same. Most of the work is in the API functions that the web and mobile are both going to be using. Serving the web page is a pretty small load on the server side, even if it runs a JS frontend.
I'm finding that Jerboa is way more snappy and has less of the "button spinning forever" or similar problems. Maybe it's just the frontend effectiveness, but still.
Are there any lemmy forks/instances running on Elixir? Rust is fine for some applications, but Elixir was made on top of Erlang, which is all about ensuring maximum uptime and speed with the least resource usage