A 12k RPM spindle can cut steel with a very small tool, under 1 mm. But it's too fast for larger ones - optimum RPMs decrease with tool diameter. Some belt driven spindles can run at low RPMs without losing torque; the answer to your question depends on the specific spindle you're planning to use. But belt-driven spindles in general are better for this use than the self-contained spindles used on routers, which generally can't slow down much without losing torque and overheating.