Immediate family of U.S. citizens are the highest priority family based I-485s.
I would expect marriage based I-485s to take longer than I-485s filed by parents of U.S. citizens, or unmarried children of U.S. citizens, at least when the U.S. citizen or child were born in wedlock. In those two cases, bonafide relationship evidence is rarely requested beyond birth and marriage certificates. The decision is largely deterministic.
Whereas marriage based I-485s require a subjective decision as to whether a bonafide relationship exists.
IMO, over 10 percent of the time, it does not exist. Mostly it is because one spouse believes there is a bonafide relationship, and the other spouse has no intention of behaving as they did before I-485 was approved.
USCIS knows they approve marriage based I-485s from con artists each day, and it wants to improve. So slower approvals.