Musar к Шмот 21:12

מות יומת הנואף והנואפת . The reason why both adulterer and adulteress are to be executed by suffocation (20,10), a relatively easy death penalty, may be connected to what the Torah writes elsewhere concerning the type of death penalty to be administered to a murderer. For a person who murdered another with one single stroke (or bullet causing instant death), the Torah legislated death by suffocation; he presumably did not make his victim suffer. There is no reason therefore to impose a harsher type of execution on the adulterer.