I listen to a few podcasts that are critical of the usage of “Well, actually…” and “Turns out…” in internet culture. With the internet comes a full deck of cite-able sources of various quality and provenance to correct people in an irritating way: “Well, actually the study that said to drink all that water did not say that”, “Turns out salt intake isn’t bad for blood pressure”, etc., etc.
This is valid cultural criticism and should be noted by people. Much of the time the value of the correction is overshadowed by the irritation it causes, and half the time the “correction” is just as questionable as the original statement.
I would submit that the “Um” prefix is the dirty bomb of such phrases. “Um” does not just telegraph a “get ready to hear something I read that you didn’t”. “Um” says, “You moron, haven’t you read that…”
Anyone familiar with Hacker News would not be surprised to see me use a link to a discussion on that site to illustrate the point. It strikes me as a shorthand for the person who says “Time out” and then corrects someone in conversation. It is both pretentious and superfluous. I struggle to imagine a situation in which the meaning or clarity of a statement is not improved by removing it from the front of the sentence.