This is often a tough one for those of us who have been in the sport, particularly road riding and racing, for a long time:
Lower pressure (to a point) is faster than rock solid.
I started road riding and racing on 21's and 23's. I can remember seeking out Vredstein tires because even with a tube, they could be pumped up to 140 psi, as opposed to many other's topping out at 125 pounds per square inch (psi). During a stage race a couple decades ago, a friend let me borrow his disc wheel with a tubular. He said, "just check the tire pressure. " I replied, "How much? Like 125, 130?" He simply laughed and said "Not quite. Pump that sucker up to 180." One hundred eighty psi on a disc wheel with my aluminum bike! To say it was a bit of a harsh ride is an understatement. I don't fault my friend. At that time, it was readily accepted that harder was faster.
While it's been accepted in the off-road world, particularly in 'cross, that lower and very specific tire pressure provides SIGNIFICANT advantages, the road world has been a little slower to adopt this mindset.
This Fasttalk podcast with the master himself, Lennard Zinn, does a great job of explaining the concept of psi and how tire size, road/trail surface, rider mass, rider style, and other important factors need to be considered.
I've posted this before, but this is a great tool to help you get a starting point:
Coach B.L. is the head coach at BJL Coaching and an avid racer and cycling enthusiast himself.