See Jeffrey H. Tigay, “‘Heavy of Mouth’ and ‘Heavy of Tongue’: On Moses’ Speech Difficulty,” Bulletin of the American Schools of Oriental Research 231 (1978), pp. 57–67. While the idiom elsewhere connotes an inability to speak a particular language (Ezekiel 3:5–6), I do not think this is relevant here, given God’s answer in Exodus 4:11, “Who gives man a mouth, or who makes dumb or deaf or percipient or blind? Is it not I, Yahweh?”