Maybe, but I can do some amazingly aggressive fast parting if the tool is up to it. I think that a limp loose machine, bad grind, and poor tool position are more likely to chatter than being a little off on speed. I'm not saying speed can't be a factor. Just that there are things one can do to increase the range at which it works ok before doing a motor swap.
In your case you made your machine less limp and sloppy.
... and of course its all just my opinion. I'm not a master machine builder.
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I bought the "Machine Tool Reconditioning" book, a surface plate, 0.0001" indicator, straight edge and cylinder square. So far I've risked improving/damaging only the ways of the AA lathe and the lower surface of the SB's compound. Maybe some day I'll take on the others. I've learned their limitations and design around them.
The book was valuable to fit the warped heatsink of a thermoelectric module controller to a spreader plate, on the space comm laser project. The heatsink wasn't thick enough to flycut flat. I figured that quietly fixing the problem myself was better than telling the project manager he chose and bought deficient equipment, after he had proudly told me what a good deal he got on it.
My shop let me assume all control over the fabrication of most projects, so that I was only rarely caught between an electrical and a mechanical engineer who didn't understand each other's field.