There is no simple formula for the speed of any mixer.  At the very least, small mixers run at higher speeds than larger mixers.  Even tip speed, which involves the impeller diameter multiplied by the rotational speed, depends on the application.  In the case of an anchor impeller, which is not a particularly good mixing impeller, the rotational speed will depend on the viscosity and process result.  High viscosity may mix better at a slower rotational speed, because the fluid needs more time to flow.
 
Asking the speed of a mixer is like asking how far up is high.

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