On Language Log, Geoffrey K. Pullum invites readers to send in the earliest citation of the use of periods to indicate slow, intense speech.
On page 28 of Robert Harris’s novel Archangel (Hutchinson, London, 1998, hardback edition), a character who was tortured for a long time to get information out of him says with pride, “Not a word, boy. You listening? They did not get. One. Single. Word.” That’s the usage I’m talking about. So it’s at least ten years old. Now, if you can find an occurrence that is earlier than that, and earlier than all the ones above yours in the list of comments below (if there are any yet), kindly supply the details.