That is not completely true. I experienced the "chatter at normal speed but too slow of feed". I would suspect that no less than 1/2 recommended speed is appropriate for parting. The problem with too slow is that feed becomes tricky, even with nerves of steel. Higher speeds give you more leeway in the feed rate. As I recall, I was parting aluminum (3/4" at 750RPM). I got a lot of chatter until I increased the feed rate. The noise changed to a smooth hiss, ribbons started flying off, and the finish was excellent, even to a professional machinist who I showed this to. But timidity does have its advantages. I am pretty smooth, and I can smoothly force the parting tool into the part until the lathe crashes. I only did this once, though. Ok, twice. So "feed faster" is not a hard and fast rule.