Get a road bike. If you can cruise at 20 on an MTB, you'll be at 25 on a road bike. I average 22-25mph on the flats with no real effort on my road bike. I did the MTB/Slicks option before I went to a road bike for commuting on roads. There's one guy on my morning commute that I think is about to crack and buy a road bike. I've passed him so many times, and he always trys to keep up, hangs for a couple blocks then drops off. Last time I went by him, his comment was "Man I need a road bike". FWIW, there's a 6 block stretch of 35mph road on my commute with parked cars on one side, median on the other, and about 9-10 feet of lane. I take the road and hold my speed at 29-30. So to answer your question, no you're not slow, MTB's on the road are slow, and road bikes are really that much faster.