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I figured it out. Project Settings -> Timeline format -> Playback Frame Rate was
29.97 while Timeline frame rate (grayed out) was 24. I changed the Playback
frame rate to 24 and now it previews normally. I didn't make any change to
neither frame rate. Something I did must have changed it. I don't get how
changing this number causes preview to ramp up in speed, but whatever.
Now I know.