Hey Tom, hey @cvcolomb,
This is true. On my spindle, a 2.2 kW 6000–24000 rpm spindle, the manufacturer tells in the manual to program a lower speed limit of 6000 rpm in the VFD. The constant torque range of a typical 24000 rpm spindle begins only at 6000 rpm:
Also when milling wood, you mill at high speeds. This is because the problem with wood is heat. In this popular jazz bass video, everything is milled at 24,000 rpm: DIY High-End Jazz-Bass-Guitar produced with the CNC machine – Youtube. Of course with a large bit diameter you would choose a lower speed, but you’d have to stay inside the constant torque range of your spindle.