Written in Ruby. Oh dear. I would have at least expected it to be in Node, old skool kids these days.
On a more serious note, my shell of choice is written in C, runs fast, doesn't require a dogshit slow interpreter like Ruby to run, and frankly, I likes it that way.
Why not try it first before doing the knee-jerk hipster "ruby is slow zomgz guys" response.
If it gets big someone will reimplement it in c or some other more not "dogshit slow interpreter" (of which you are aware of rubinius and jruby right? they don't really interpret anymore) language.
And from 2 years ago when it was first introduced to HN: https://news.ycombinator.com/item?id=4512928