Phoenix has significantly faster templates than Rails by compiling templates and leveraging Erlang's IO Lists. So you will basically never think about caching a template in Phoenix.
Most of the Phoenix “magic” is just code/configuration in your app and gets resolved at compile time, unlike Rails with layers and layers of objects to resolve at every call.
Generally Phoenix requires way less RAM than Rails and can serve like orders of magnitude more users on the same hardware compared to rails.
The core Elixir and Phoenix libraries are polished and quite good, but the ecosystem overall is pretty far behind Rails in terms of maturity. It’s manageable but you’ll end up doing more things yourself. For things like API wrappers that can actually be an advantage but others it’s just annoying.
ASP.NET and Springboot seem to only have theoretical performance, I’m not sure I’ve ever seen it in practice. Rust and Go are better contenders IMO.
My general experience is Phoenix is way faster than Rails and most similar backends and has good to great developer experience. (But not quite excellent yet)
Go might be another option worth considering if you’re open to Java and C#