I struggle with how to say "both Ruby and Rails are poor technology choices" without sounding ranty. I have seen nothing good come from the flexibility and non-portable conventions of these tools. I have seen so much nicer web stack designs than Rails, with none of the problems. Maybe if you're building a Basecamp type site - slow / speed doesn't matter, limited views / minimal front end functionality, small surface…
GitHub, Airbnb, Shopify, Indiegogo, Kickstarter, Urban Dictionary... the list goes on and on and on. Having the hardline opinion that Ruby or rails is slow is plain ol’ ignorance. You can build highly performant apps in Ruby and Rails, you just need to know what to optimize. The same applies to any other language or platform. Facebook is still humming along on PHP.