For concurrency, async fibres are brilliant but not the only game in town.

Using good old processes works really well in Pitchfork or alternatively JRuby and TruffleRuby both have true non-blocking parallelism right now using Threads.

If anything TruffleRuby shows that there is nothing inherient in the Ruby language that means it cannot catch up in performance terms.

My personal dev speed however is a little slower in Phoenix than Rails. And I've been using Rails for almost 20 years and Elixir for 10.

There are categories of problems that are much easier in Elixir (High concurrency, low latency, etc) and for those I would always pick Elixir. If you're planning "just" a crud like web application, Ruby will probably get you to product-market-fit faster. There are (well maintained) gems for almost everything.