You need speed for turning. To give an example, I run my lathe at 2000rpm for 20mm aluminium, and that is nowhere near the recommended speed. The reason I limit to that, is to stop the stock whipping in the spindle, as it's not got any spindle liners to support material.

Running some basic figures for 20mm bar through HSMadvisor, for 6061 series aluminium recommended speed is 10222rpm, low carbon steel is around 1900rpm, and 316SS is 724rpm.

With stepper motors, the rated torque is holding torque. As soon as the motor starts spinning, the actual produced torque at the shaft quickly drops off.
Also, you can only guarantee a stepper motors position to one full step (1.8 degrees for a standard 200 step motor). The main purpose of microstepping is to improve smoothness at low RPM, not to improve positioning. Microstepping may provide fractional steps in movement, however due to friction/stiction within the system, the exact distance cannot be assured.