I'm not an expert in mountain bikes. My understanding is that mountain bikes are made of heavier tubing so they can withstand off-road forces. The geometry is different (things like higher bottom brackets for clearance) longer chainstays? which greatly affect the handling and steering of a bike. The wider tires have more contact with the road so they increase resistance slowing you down. They also typically use flat bars which dont allow you to get low and out of the wind and i personally feel much more confident going down a big hill at high speeds of i can brace myself in the drops when i'm braking.

Take a nice racing bike for a test ride at your LBS. For commuting you would probably want something more cross or touring oriented as they allow for racks and fenders etc but for test rides, why not experience the difference.