Goddess Nemesis was not going to allow any of her slaves to be rude and when she noticed that this one was, she had to act and she did so in a painful and humiliating way. The mistress laughed at the guy and she made sure that he learned his lesson in a way he would never forget. By the time she was done with him, his rudeness had evaporated.
Goddess Nemesis was out to control this loser and she had to do it to the best of her ability. She chose to do it with her faceslapping fetish and that is why she cruelly slapped the shit out of him. He did not think she could do that but she did and the point was home. He wished that he had done things better but he did not and for that, he had to pay the price.
Goddess Nemesis wanted this guy to stop being slow and she used her hard slaps to dominate him. The mistress was tired of how slow he was and she felt that by keeping quiet, she was enabling him. That is why she opted to cruelly torture him and she did it in a way he would never forget. He was in pain as she landed the hard slaps on his face.