First off, a concession: Yes, Ruby is slower than many languages on many benchmarks. However. “Slow” is a useless descriptor if you don’t have a “fast enough.”
Allow me to strain an analogy. If you’re a driver, I’d bet that whatever car you drive isn’t as fast as what they take on the F1 track. There’s a good chance that you still consider your car fast enough. If you do have a super fast car, you might brag to your friends about how it does zero to sixty in 0.036 seconds, but 99% of the time you’re never going above the posted speed limit.
At some point you start asking more relevant questions like how many cupholders it has.
… that contributors to the Ruby language have made excellent progress on Ruby 3x3, the goal that Ruby 3 should be three times faster than Ruby 2?
… that there are Ruby implementations like JRuby and TruffleRuby built for concurrency and speed?
… that Ruby’s got a snazzy new JIT compiler that’s leading to some pretty nice gains?
… that Ruby 3 introduced Ractors, a handy implementation of the Actor Model for concurrency?
GitHub is doing fine. GitLab is doing fine. Shopify is doing fine.
Top Ruby Companies indicates that, at the time of writing, the last 12 months have seen an overall revenue of $61.5 billion dollars for companies that make Ruby a key part of their stack. That seems like scale to me.
You can do a heck of a lot with Ruby other than web development. Ruby is a great choice for plenty of software, like…
Package managers like Homebrew. Local deployment configurations like Vagrant and infrastructure management tools like Puppet or Chef. Hey, why don’t you write something like Fastlane to automate huge swathes of your mobile app deployment pipeline?
Shoes helps you build GUI applications for the desktop.
You can make games in Ruby using tools like Gosu or DragonRuby, and Ruby is used as the plugin language for some versions of the popular RPG Maker engine.
Metasploit modules are written in Ruby.
Drop beats with Sonic Pi.