Motor RPM is Kv * effective voltage
while the effective voltage is battery voltage lowered by current * resistance (battery + ESC + winding resistance)
means motor RPM = Kv*(U-Ri*I)
so if your motor needs 40A and its internal resistance is 50mohm, then effective voltage is at least 2 volts lower. Means it makes 20% lower RPM on 10V battery
BTW it has exactly same rules like DC brushed motors