Belts tend to run better when tight. Actually, they like to be tighter than a chain driven option to prevent slipping. This one appears to be mighty slack, so I'm guessing that as one settles into sag that the belt tightens up?
Horrible bike but i think this new 'chain' idea is quite cool as i snapped 3 chains last weeks and 2 on my other bike before, so good to see something a little more advanced