Was Moses "slow in speech" or "mighty in word"?

In Exodus 4:10, Moses complained of “slowness of speech” but Stephen in Acts 7:. says he was “mighty in words and in deeds.” Is there not a contradiction here?

Someone has said that Moses took forty years in Egypt learning all the wisdom of the Egyptians, then forty years in Midian learning that he knew nothing after all, and then forty years in the wilderness in learning what God could do with a man who had learned he knew nothing. There is no contradiction here, or anywhere else in the Scriptures. We read first what Stephen says of Moses, that he was “mighty in words and deeds,” that is, during his Egyptian period; then what Moses says of himself. Perhaps there are good speakers to-day who are genuinely diffident, and are quite unaware they are eloquent. It does not matter, if they think so, as long as they are all the more cast upon God and fulfil their ministry. Possibly the reverse is true of some of us. We think we can speak and would do well to be more often silent. In fact, the word is addressed to all “Let every man be swift to hear, slow to speak, slow to wrath.”