Sure sounds like 3/4" supply to me.
I have 3/4" PEX coming from the water main, thru a Watts 3/4 in. Cast-Bronze Pressure-Reducing Valve, thru a ball valve whole house shut off and continuing to my water heater.
OP says the problem did not exist when new. If that's the case, a clogged screen in the pressure regulator, or some other problem with the regulator, is much more likely to be the root cause here than pipe sizing.