Electric unicycle? I own several. Some will even go over 50 km/h, though I don't dare go that fast. It's just safe to have the extra power when needed.
I asked about electric uni not for speed btw. I've seen a few and people were faster than traffic even at moderate speed (in city of course), looked relaxed as fk, the device was small. I don't know about energy efficiency but I was tempted to drop all other mode of transportation after seeing that.