All tires have an ideal pressure for the conditions they're being used for. My mountain tires are typically inflated from 35 psi (beefy knobbies on rough trails) to 70 psi (narrow semi-slicks on pavement).
In general, higher pressures mean lower rolling resistance. However, there exists a point where higher pressures actually result in higher rolling resistance. It takes energy to lift a bike up and over a bump in the road and unless you're at the velodrome, at some point the energy being used to lift a bike up and over the bumps exceeds the energy being lost to tire deformation.
The difference in speed vs. pressure vs. terrain is especially obvious on my cyclocross bike. On a race course, 35-40 psi rules as it sucks up all the surface irregularities and allows you to ride faster. On the road, 35-40 psi is like riding my mountain bike whereas 65 psi is actually pretty quick.