I ride an 86 RockHopper with street tires and an "opgraded gear set. A new bike would probably be a little lighter and faster, but since I'm limited to a very upright riding position because of a bad back, I can't really justify the expense.
Spend a few $ on skinnier tires and MAYBE a cassette that MIGHT work better for your use and keep riding.
Those changes on my bike has allowed me to raise my "cruising" speed from about 10 to 13 MPH.