Yeah, 6.2mph is silly even just from a riding perspective. I am not the most fit and I'm fairly sure I would need to force myself to ride that slow (and it'd probably feel less stable). Around other people sure.
12.4mph is not so bad, at least with my underpowered ebike I'd have to really put in a lot of power to go above that and it'd use up too much battery power (and thus range). If anything I stay around 9/10mph for efficiency (and it's just a comfortable riding pace, also I have an upright riding position so not the most aerodynamic). My ebike's motor cuts off at 15.5mph (even though again, it doesn't really go that fast) because EU rules (even though I'm in the US).