Lift the front wheel off the ground, and spin it, how long does it spin for? Can you hear or see the brake blocks rubbing against the rim? Are the bearings making a horrible noise?
If not, lift the back wheel off the ground, and give it a spin with your hands (if you use the pedals, it normally goes too quick to notice anything), and see if any of the above is happening.
I don't want to sound rude, but 7 mph is TERRIBLE, I could easily average that when I was 4, so there must be something terribly wrong with your bike!
With a tail wind the other day, I managed to go nearly 30mph with no effort!