On the slowness, I would agree that Ruby is slower than many languages, but as an interpreted language this is -up to a certain point- by design, or at least an accepted part of the trade-off one accepts when he takes a programming language. Moreover, Ruby's natural competitor is Python, not -say- Clojure; and I wouldn't that Ruby is significantly slower than Python.
I use Ruby for everything I can, Python for ML, and R for data analysis. I just find it easier to not swim upstream and force ruby onto all of those contexts when the other 2 languages have loads of inertia behind them in those spaces. It's unfortunate.
Why problematic?
On the slowness, I would agree that Ruby is slower than many languages, but as an interpreted language this is -up to a certain point- by design, or at least an accepted part of the trade-off one accepts when he takes a programming language. Moreover, Ruby's natural competitor is Python, not -say- Clojure; and I wouldn't that Ruby is significantly slower than Python.