That’s a pretty low bar to set though. 
Only PHP 7 started getting better and faster than the nightmare days of PHP 5.0 - 5.4.
I’ll respectfully disagree that it’s not a resource hog. I do agree that you need to get to a certain amount of users until Rails becomes problematic to scale though. But, that number is still lower compared to Phoenix.
I worked with Rails for ~6.5 years on ~15 apps (if memory serves) and I have two apps under my belt that I ran concurrently both as their Rails and Phoenix variants with 100% feature parity (since rewrites were approved and I had to gradually migrate everything without stopping the Rails apps in the meantime). I am probably a part of a very small group of people around here who have the practical and direct experience of running identical Rails and Phoenix apps (backed by full nights of setting up telemetry and measurements together with sysadmins). In both apps (pretty normal commercial CRUD + API + some bells-and-whistles-attached apps) the average latency was 9x - 12x less in Phoenix, and it also used 3x - 7x less RAM during most loads, extreme included (anywhere from 100 to 7000 users a minute). The DB was 100% identical and its configuration was never changed during the Phoenix rewrites either. The hosting VPS-es were all identical as well.
TL;DR: When the rewrites were approved, trust me, me and the team did all the due diligence to prove that the rewrites are saving hosting money and are allowing for further growth of the business. Those points were part of the contractual agreement so we paid special attention to them at every step.
I appreciate the call for being diplomatic but I feel it goes a bit too far when technical arguments and experiences get swept under the rug by “all frameworks are awesome”. No, they really are not, and me and many others have the battle scars to prove it. I am sorry that this offends someone but I am also not willing to accept that years of my professional experience have been a fantasy.
I am however not bothered enough to go on the net and dig the numerous proofs that Phoenix is more lightweight [on system resources] than Rails. Hence, I should have probably never started this argument to begin with. 
I recognise that and I am sorry that I got slightly worked up. I hope you do somewhat agree with me that blatantly discounting other’s experiences with “Phoenix is not mature” – and refusing to elaborate – is not a constructive thing to say either.
Fully agreed on that. I will not comment further on this sub-topic that me and @jslearner started. Apologies that it was viewed as a derailment of the main topic.
