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Slow down too much, and you wind up work-hardening the steel, and wearing out the end mill too rapidly. And even at the slowest, the forces cause lateral deflection to the left.
Actually -- CNC would look better than manual feed, because the feed rate would be constant, so the deflection would be pretty constant except at the very start and end of the cut.
Another advantage of the two flute and three flute ones (and some four flute as well) is that they are ground to cut to the center, so it is possible to feed directly into the workpiece. (A drill bit feeds faster, but it is possible to do it with the end mill. Any one that I have seen above four flutes (and some four flute ones) will not cut in the center, and thus cannot be fed directly into the workpiece, unless the center has already been drilled out.
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That is a start.
Enjoy, DoN.