For smaller sized holes, such as 1/2″ or smaller, the maximum suggested speed is 600 RPM.
Larger holes will need slower drilling speeds, for example:
400 RPM for 1 inch Holes
200 RPM for 2 inch Holes
125 RPM for 3 inch Holes
The idea is, as the speed increases, there is more and more friction on the rim of the drill bits and the coolant can not stay on the surface.