These kind of vehicles are very fast. I own a Mango, that is not as fast as the Quest, but still one wants to yell "yihaa" or something like that when going downhill.
Where I live there are not big or long hills. It is gentle rolling landscape, which nevertheless can be quite thrilling at times, especially when the roads are narrow and twisty.
My speed record is not very fast, 61 km/h, but that was just coasting with no pedalling.
/Bruno