Phase control universal motor dimmer
A phase control circuit is the simplest way to provide reasonably good speed control for a universal motor. Controlled rectifier control would probably be a little better than TRIAC control. A full-wave rectifier with a PWM control could also be used. That would probably be a little better than a controlled rectifier. How much difference there is between one method and another and exactly why one method might be better that another is hard to say, but a comparison can probably be found somewhere.