I don't know what type adjustable regulator you have, but I hope it is a good one. Most devices (e.g., CAMCO's) also severely restrict water flow rate, but a residential type like the Watts flows the same at any pressure. I went with this fixed 55 psi regulator as a cost compromise with the Watts unit
https://www.rvwaterfilterstore.com/Fixed.htm We use it in conjunction with a Rain Bird test gauge to check the actual pressure.
So if you're disappointed with the flowrate, consider such alternatives.