I have this exact same problem. Just bought a new house and it has a moentrol setup in the master with a single head and 4 sprays. I am working backwards to determine that it must be a moentrol because it uses the "263ORB Kingsley trim for Moentrol" (can't post a link to the sites that I found selling this).
Anyway, all the other showers in the house that just have a single head (no body sprays) flow between 2 and 2.5 gpm. The head on the moentrol setup flows 0.65 gpm! The body sprays are very forceful, but the head is pathetic, even when it is the only thing running.
Is there something that can be adjusted/replaced inside the valve to fix this? The shower is covered in a pricey travertine so I'm really hoping the fix wouldn't require getting "behind the scenes".