1) Elixir + Phoenix have some definite advantages when spawning asynchronous tasks (no external dependencies, it's in the language) and it runs reasonably quick.
2) Coding in Ruby on Rails is faster. An example, it's much easier to write queries in ActiveRecord than in Ecto, which is over engineered for the common needs of a web app. Queries, updates, transactions, everything are productivity killers there. For 99.9...% of the web apps it will be fast enough and the CPU will sit idle most of the time.
3) Django is as bad as Rails for concurrency and performances. The ORM is more verbose than ActiveRecord but luckily it's much closer to AR than to Ecto. The project structure is very loose compared to both Rails and Phoenix, which is bad for onboarding people coming from other Django projects ("ah, you put that there, why?")
My suggestions. If you know you're going to build a big application, scale it, etc, use Phoenix. Small to medium size: use Rails unless you're already a Python shop. In that case use Django. Smaller frameworks like Sinatra or Flask: it's not worth using them with the exception of very small applications that won't grow bigger. If they do you'll start adding components and turn them into an homegrown nightmare resembling the more polished Rails and Django.
