If backend speed/scale is your concern I'd also throw a vote in for Phoenix. Lots of similar heritage to Rails but built on Elixir/Erlang instead.
Unless that's changed since I last tried Phoenix, I'd be hesitant to use it in the same niche as Rails.
But I do agree that losing the huge ecosystem of gems is a significant downside. Hopefully that'll change over time, because other than that, Phoenix really feels like the next iteration of Rails to me.