First off, I would NOT recommend trying this.....
That said, a guy I knew had a combination mill/metal lathe that had mechanical, but limited, speed control, with an induction motor. He put one of the router speed controls that you can get cheap at Harbor Freight. He makes sure it is set a full speed, turns the machine on, and proceeds to slow the motor with it. I was amazed he did that with out smoking the motor!
So, while it TECHNICALLY worked, he is very likely adding a LOT of extra heat to the motor's windings. I can't imagine that will continue to work as a long term solution for him, and if I were to guess, he will end up with a bad motor of continued use with it. Not to mention the drastic decrease in working power of the machine.
So again, I would NOT recommending putting a speed control on a universal motor, as you risk permanently destroying it.
The start circuit in an induction motor is normally engaged any time that the motor speed is below about 80% of it's plated speed. The speed of the motor is primarily a function of the supply frequency, with the motor load having a slight second order effect. Increasing or decreasing the voltage applied only affects the current drawn, lower voltage means higher current needed to produce the same power, means more heat produced in motor windings, leading to baking the insulation. That's why a lot of moderate to high power tools come with recommendations about not using them with long (or any) extension cords. The voltage drop in the cord an be underestimated by the lay person, which results in a low supply voltage to the motor, excess current draw through the cable, increased voltage drop in the cable, in an every repeating cycle until a breaker trips, cables fail, or the motor bakes.