[jackjr-123]

I took that off the manufacturers data that accompanies the spindle. The Torque/Speed curve clearly indicates 10,000rpm as rated speed.
It is four pole, ie 333Hz is 10,000rpm, 700Hz is 21,000rpm and 1333Hz=40,000rpm.

That sounds right. I missed the part where you said it's a 3.5kW motor. Mine is 5.5kW, hence the confusion.

Even if the torque is enough for some steel milling, you also have to keep in mind that the bearings stiffness in these spindles is lower due to the special geometry to allow these high speeds.

Just for reference, a small benchtop mill with gearbox and a 1.1kW/4 poles async motor has about 100Nm at 100rpm.