I, too noticed a BIG change when I swapped out my stock knobbies for slicks. I picked up about 2-3 miles an hour and my cadence has really risen.

I run my slicks at 90-95 lbs air pressure (they're rated for 100) while the old knobbies were rated for 60 and I ran them at 65.

One of the things I do before every ride is check air pressure and pump my tires. Riding on soft tires sucks.

If you are mainly road riding, get slicks.

Invest in a good floor pump WITH gauge--Keep the tire pressure up where it should be--it makes a big difference.