I heard the quote in passing but can’t find it. It was translated from a cuneiform tablet something to the effect of “a skilled cuneiform scribe can write as fast as the mouth moved”

Dr. Irving Finkel has said the same. The Babylonians and others managed massive bureaucracies at the time handling thousands of letters, court transcripts, and official dictations. There was more than enough practice for a scribe to get fast at it.