As to why it is slow I do not have the knowledge to tell you, but concerning why you should use it, well maybe because it's the only company-backed "Ruby on Android" solution at the moment?
Looking at open source world instead, there's also Ruboto which leverages JRuby (a Ruby implementation running on JVM). It is currently focusing on supporting JRuby 9000, which would represent an even more interesting speedup. However I'm not sure it is optimized for a speed test yet. Nonetheless, it would have been interesting to see it compared with other solutions in the writeup...
I found the free packet a little bit to late in the market.
Anyone from RubyMotion dev maybe can explain way is so slow, and I should use it instead of robovm, swift, xamarin, etc.