Sure he is doing an ok job sucking on her feet, but he can do better than that. He needs to show more vigor, more enthusiasm, show her how grateful he is to be allowed to worship her feet. After all Halle doesn't grant such a privilege to just any slave, her feet are a reward for loyal slave service. So he needs to lick them faster to bring a smile to Halle's face, to show her how much he loves living in her service.