Given that the Sun rotates, and given that it rotates sometimes slowly, sometimes swiftly and sometimes at a moderate speed, would this not be the reason for The Quiet Sun rotating slowly? Makes sense I think. Have a nice day.
The suns rotation speed does not vary significantly. It rotates at difrrent speeds depending on latitude though, about 25 days at the equator to about 36 days at the poles. This differential rotation speed does appear to be related to the periodicity of the 22 year sunspot cycle.