With cast bullets, fit is king. Everything else takes second place.
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I don't understand why some would get the leading at low velocity and with hard cast bullets and others don't.
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If the bullet doesn't properly fit the bore, or in the case of a revolver, if the proper relationship between cylinder mouth/forcing cone isn't right, then the bullet will lead the bore. A soft bullet at low velocities might obdurate sufficiently to reduce leading, it might not. The key is to have a proper fit of the bullet in the bore.
I normally shoot dead-soft lead up to about 1000 fps, go to a harder alloy at 1000-1500 fps, and use my linotype for bullets that will go past 1500 fps.
Richard Lee and Hodgdon powder did some interesting work with pressure several years ago and tried to define the relationship between pressure, bullet speed and bullet hardness. Lee added that research in one chapter of his book, Modern Reloading. It's well worth the effort to get a copy if you're interested in those relationships.