Hey! There has been many changes to the game since that demo was made and for many who experienced slow downs with the demo, it was not an issue with the full version anymore. That was the case with myself too. And then people who suffer now with the slow down often didn't have the problem with the demo. This suggests that it's a new and different reason that causes lag in the full version.
In any case, you can buy the game if you want and if the problem still remains with the full version, I can always refund it.
Otherwise I will continue looking into the problem and hopefully find some solution to all this.