Several possibilities here.
The water heater usually will have a check-valve on the back (maybe two.) One on the cold input to the water heater and one on the output hot side. These allow one-way water flow and are famous for plugging and jamming. I think these are what "Old Guy" was talking about in his response.
The bypass valves (used to isolate the water heater from the water system) are for winterizing with "pink" and saves you from needing 10+ gals of pink because its filling the heater. There are many cases reported of these valves plugging up with gunk and being 90% closed even with the handle at the open position.
Bottom line: if only one faucet is slow then it's the faucet (screen/line etc.) If everything is slow then its the water heater (check valve, bypass valve, water line) assuming you have good pressure on the cold side.
2004 Winnebago Vectra 40KD
Roadmaster Tow Dolly