Hi Holofonoro, thanks for the interest :)
I do plan on bringing the demo back; I removed it not long after the Steam release. I very much want to get a demo running again but, as the game is in Early Access, I update almost daily and don't have the time to also keep a stable demo build available. And I'm not comfortable with providing an outdated version. Once things calm down I can shift my focus back because demos are very important for games to have.
That said, the game not running well on your machine is a perfectly valid reason to ask for a refund that I would be happy to provide.