That being said, I agree that performance for certain IO intensive applications like web servers is very good, way better than Ruby. Subjectively Phoenix feels way, way faster than Rails in development and is much more performant in production.
Yeah, but how many companies do you know of that perform heavy computations in such large scale that their choice of programming language has a noticeable effect on the result?
I mean, it might matter for tech giants like Google, who need to squeeze every bit of processing power out of their CPUs. For almost everyone else, it will be a non-issue.
Going from Phoenix to Rocket.rs I saw a pretty dramatic speedup on even simple pages. Phoenix was faster than the Rails version of the app, but still slower than a compiled language.
The BEAM was originally designed to run software for telephone switches. It's often referred to as soft realtime because it is so responsive. It has been battle-tested for now over 3 decades.
Where the BEAM falls flat is pure number crunching but it's blazing fast with binary processing (e.g. string parsing).
A fresh phoenix project where you render a nontrivial template easily reaches sub millisecond response times, without any kind of optimization.