The answer
Single fixture drops are caused by clogged aerator screens, mineral scale on showerhead restrictors, or failing cartridges. Whole-house drops point to partially closed shut-off valves, failed Pressure Reducing Valves (PRV), or supply main leaks. Residential plumbing codes mandate delivery pressure between 40 and 80 PSI; below 40 PSI indicates regulator or delivery failure.
Sources
This answer is drawn from the standards and guidance below. It is general information about how these systems work — not a diagnosis of your property.