I run 700x32 @ 65 psi front, and 700x28 @ 70 psi rear. Why so low? Because the average speed penalty is only 0.5mph vs. the higher 90 psi. On the plus side, I get much better traction/handling, and a more comfortable ride.
There are three major factors that limit my average speed:
1. Traffic...can't control
2. Road condition...can't control
3. Road hazards...can't control