Like I said earlier, the RPM drop under load is not important because it should be handled by correct configuration of the VFD or the actual milling process. If your VFD is in sensorless vector control mode then the VFD will keep the RPM necessary for the load to keep a constant torque. You need to tune the VFD to your motor to get optimal functionality. Auto tuning might work, but in my case I had to adjust a few parameters manually. Unfortunately, I don't remember which of my parameters were incorrect after auto tuning, but it was not very difficult to get it right.