Diode only speed control of DC motor
A series of diodes will create a series of stepped voltage drops. The voltage drops will not change proportionally with current as the voltage drops across resistors would. That means that there will be less speed variation with changing load. That is an advantage over series resistors. Using diodes in this way still causes heat and inefficiency similarly to series resistors. If the motor requires a maximum of 2 amps and the voltage drop is 1 volt per diode, six diodes would dissipate 12 watts and the motor would dissipate another two watts or so in winding resistance, friction etc. leaving about 10 watts converted to mechanical energy with an efficiency of about 42%. That is a pretty low efficiency, but only 14 watts dissipated as heat.

The same technique could be used with a larger motor, but the energy lost as heat would increase proportionally. With a 1/2 Hp motor, a similar setup would use 888 watts and dissipate more than 500 watts of power as heat to provide 373 watts of mechanical power.