Phoenix is faster than Rails, however Ruby is faster than Elixir when doing CPU intensive tasks. Like parsing text.
What is your definition of slow or fast?
For example with Elixir it was possible to generate 5,000x random 19 character codes in 3ms on a 5 year old i5 3.2ghz desktop workstation. I don't know how long it would take with Ruby but for such a text / CPU intensive task, I'm quite happy with the performance.
Also parsing templates with EEx is ridiculously fast, because it's not treated as a string.
spec-builder git:(master) ruby -e "require 'benchmark'; puts Benchmark.measure { 5_000.times { rand(36**19).to_s(36) } }"
0.005459 0.000009 0.005468 ( 0.005464)
It would be interesting if you wanted to come up with a Ruby version that adheres to the random code spec which should be able to be determined from looking at the first 2 versions. It's basically only a subset of characters that can be used and the code should be output as XXXX-XXXX-XXXX-XXXX.
I personally use the 25ms version because I find it to be the best one if you factor in both speed and readability. I'm sure there's better ways but I'm only an Elixir beginner.
Also for the record I didn't write the 2nd or 3rd versions of that gist. I asked around in the community and that's what others have put together.