If you are using carbide tools to thread with, you most likely would want to thread in steel at around 300 to 400 surface feet per minute. Cutting speed is fairly important in steels because of the "critical speed", which is the speed at which the chip flow is the best. Generally speaking, when turning at or above the critical speed, the turned surface is smooth and shiny, but if you are below the critical speed, then the surface is rough, and the insert suffers greatest wear and abuse from built-up edge occurring continuously on the insert.
This translates into some pretty high rpms for small diameter work, which may be above the synchronization speed of your machine with the spindle encoder. So you may have to settle for whatever you can get. Generally, thread at the same speed as you would rough turn at.
Threading aluminum, well there is no speed limit. Fast is good.
Use a declining depth of cut on successive passes, to reduce the chip load and heat on the final passes.