How is it that a kind and loving God would be slow to hear the cries of his children? Why does he not respond immediately? We must remember that the Lord works on multiple levels, and that this particular promise of being "slow to hear their cries" comes only after the second failure of the Noahites to repent. They do not repent sufficient to prevent their bondage, and they repent of their bondage at first, because of the bondage, not because they have truly learned to turn to their Lord.
The social transformation of the faithful people of Zeniff into the idolatrous people of Noah is a great change, and the seduction of the apparent wealth and prestige of Noah's kingdom (bolstered by real or imagined "victories") would have been strong. The repentance of that people requires a denunciation of that new culture/religion, and a dedication to return to God. As with the Israel under Moses, it took time to remove Egypt form the hearts of this smaller version of Israel in a different kind of bondage.