Well pump manufacturers publish "efficiency curves" for well pumps, such as this one (the curve that is most sharply convex down, whose Y axis is read from the "% EFF" scale on the right hand side):
The well pumps are usually sold with a name that reflects the flow rate that yields peak efficiency. So this one is called a "33GS" because it is most efficient at 33 GPM.
These pump curves typically presume a motor speed of 3450 RPM. How might I expect the curve to shift or stretch if the motor speed changes? I'm particularly interested in the range from about 2800 RPM to 3600 RPM.