Which kind of bike is faster: Mountain Bike of City Bike?

Since boyhood I have always used a mountain bike despite the fact that I always road it on the sidewalk. I prefer a mountain bike because it looks better to me, I like the handle bar design better, and I like being able to switch gears. I always noticed however that I was going slower than people in the street on... show more Since boyhood I have always used a mountain bike despite the fact that I always road it on the sidewalk. I prefer a mountain bike because it looks better to me, I like the handle bar design better, and I like being able to switch gears. I always noticed however that I was going slower than people in the street on city bikes. When I buy a bike in the future, I want to buy a mountain bike but I will rarely ride it off road. Should I just buy a city bike for the more convenient speed or are they not that much faster than mountain bikes?
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