Without installing RMX-OS (because it's too much of a hassle for me to bother), I can only look at the script and give you my best educated guess.
It seems like this code was written poorly when it comes to efficiency. In his update methods, he is constantly creating a new Sprite object, which, if this happens every frame, will be a heavy hit to performance. I don't know why it's doing this, but you can easily get away with just creating the Sprite objects on initialize (and performing whatever bitmap and positional changes to them) and then remove all the
Sprite.new calls in the updates. Then the update methods only have to worry about updating the sprites' bitmap, or turning its
visible flag to false when not in use.
I will warn you though, RMX-OS is not for beginners and will require a good understanding of Ruby if you want to truly utilize it.