Why don't you try it? If it is unsatisfactory, you'll know why. I run
my 3 ph 3/4 hp drill press motor at 5 Hz sometimes. Indeed, the torque
is a lot less than if I'd changed the belts, but it's ok. If you do run
it slow, for extended periods, you should put independent cooling on the
motor. E.g., a muffin fan.

Bob