I went thru this exercise with a different, relatively high torque, application from another hobby.
Speed control for AC motors, without sacrificing much torque, can generally only be achieved with a variable frequency drive ("VFD") type of controller.
Reducing from 60Hz to 40Hz, for example, will cut speed by 1/3 but VFDs will increase output from 120VAC to something higher (not sure if linear... 160VAC??)
The increased voltage and reduced frequency will reduce speed without losing magnetic saturation in the windings, resulting in roughly even torque output.
"Chopper" type speed controls, as this HF almost certainly is, reduce voltage rather than frequency and motors very quickly lose torque... You may find only a few percent of useful speed range before the torque falls off to unusable levels.
When I get time later today I will try to find cites and references, and now that I'm thinking about it, techniques used in variable speed A/C hand drills.