The HWH tank has (at its output port) a check-valve....so that water cannot flow backwards into the HWH. This check-valve causes the HWH output flow to be easily restricted (flow restriction) based on any debris in the HWH. Calcium (or suspended solids) from city water will cause this check-valve to clog.
So if in "bypass" and you have good water flow at hot-water faucets but poor flow when the HWH is not in bypass mode, it's time to become a plumber and investigate this valve.
Robi, Dale, and "The Duchess" (lap kitty)
2004 Monaco Diplomat 40' PST
Tow: '08 Buick Lucerne