Most valves will put out closer to at least 3gpm to the shower arm so that there's enough for the flow restricted shower head to max out. So, with only 1.5gpm, nearly any shower head would seem weak. For that to happen, there must be some restriction in the line somewhere. From here, can't tell you where. Some valves have in-line or internal filters, it could be in the cartridge, or maybe some debris that got caught up along the path. If there are in-line shutoffs to the valve, one of them may not be fully opened or the washer (if the design uses one) has started to come apart, partially blocking the flow.
Does the maximum flow rate vary based on the temperature? Say you tried all cold, or as close to that as your valve will allow, does the volume increase? If there's any galvanized pipe or fittings in the system, the hot side will tend to corrode sooner than the cold. When it gets bad, the internal rust can nearly block the flow. Do you get a spurt or constant rust colored water when you turn it on?
People often confuse volume with pressure. You appear to have a volume problem. The actual pressure through a soda straw versus a fire hose could be the same, but you'll get lots more flow through the fire hose (assuming the supply can handle it). The dynamic pressure drops when there are more openings than the supply can handle (or, there's a lot of friction in the supply line like from excessive distance or the number of fittings, not the case here). That's what you're seeing, the head is designed to flow more water than there is. The rudimentary problem is volume deficiency, not pressure.
