This is definitely suspicious. Phoenix in dev is going to do code reloading and other things. If you are getting the same performance in both, there is definitely something else at play.
I would expect the default Phoenix stack in production to be around a 10% impact on a pure Plug stack. Also remember that logging has a huge impact in performance, make sure both apps are logging about the same amount of information.
If results are still suspicious, please let us know how you installed Erlang and which flags were used during the installation.
