You can't talk about pressure independent of tire width and vice versa, they are inextricably linked. 30 psi in a 23mm road tyre is effectively flat. 30 psi in a 2.3" mtb tire is reasonably well inflated or even over-inflated depending on conditions. Weight is also a key factor, a pressure that works well for a lightweight rider and bike can be completely unsuited for someone much heavier. I therefore tend to avoid giving specific recommendations for pressure unless I know all the relevant factors. Still, any given tire/weight/pressure combo sits somewhere on the continuum from barely inflated to rock-hard.
My experience is that running at high pressure is only a benefit if you are dealing with exceptionally smooth road surfaces. As soon as you introduce any irregularities lower pressures roll better. Even something as small as a line of paint can be a noticeable jolt on a rock-hard tire while a softer tire cruises smoothly over. I used to run high pressure over some seriously bumpy and potholed roads, the bike (and me) would be bounced all over the place and rattled to bits. As soon as I dropped the pressure those same roads became considerably faster and safer to traverse. A soft squishy tire can seem inefficient and draggy on smooth tarmac but over anything rough the way it insulates you from shocks and jolts more than makes up for that. If you've ever been running very high pressures and hit a bump bad enough to momentarily bounce the wheel(s) up off the ground, that's directly sapping speed from you since the bike is meant to be going forwards, not upwards. Even if the wheels don't break contact with the ground every bump and jolt you feel is draining some power. A softer tyre may 'waste' power by deforming but under many circumstances that's more than made up for by allowing the bike to move forward smoothly. There's a reason you don't see anyone running very skinny high pressure tires off-road, in fact you get the best results by going in the opposite direction with very wide tires running at low pressures. That applies just as well for dealing with rough surfaces on road. You might think as I used to that by running high pressure you get a harsh ride but get speed in return, when in fact by lowering the pressure you gain comfort and speed/efficiency.
Since the roads and paths I deal with are riddled with bumps and holes I've taken to running all my tires as soft as possible. The lower limit is dependent on avoiding pinch flats and on avoiding the tire folding sideways under cornering. On 26x1.75 Marathon Pluses I run 30-35 psi front and 35-40 psi at the rear depending on how much load I'm carrying.
