For your first question the mysterious key has no use in the demo. It's simply a teaser item.
As for JoiPlay, anything related to it is completely out of my control so I can't really speak to any performance issues encountered through using it. On my PC the game runs at a pretty consistent 60 fps. My 12-year-old Mac laptop is closer to 20 but I've personally never even tried it on a phone, nor did I build it with that really in mind.