I plan on using my hole saw to cut into iron. What RPM should I use for best results?

Trying to cut metal too fast will overheat the material being cut, which 'heat-hardens' the work and causes the cutting blade to work harder and wear out more quickly. To answer your question we would need to know what size hole saw you are using. Every hole size requires a different RPM for different types of materials. IRWIN always details the optimum drill RPM on each hole saw package. Before you use your hole saw to cut iron, please refer to the manufacturer recommended RPM for the appropriate hole size.