The answer as to why the speed adjustment air motor performed so
slugglishly, is, as Dan suggested, that the outlet air muffler was
plugged up with crud. I took the muffler off and without it, the motor
adjusts really fast! The muffler was, surely enough, almost totally
blocked. It did not matter for brake, but did matter for the air
motor.

The muffler looks kind of cool and old style, so I will try to reuse
it.

I am cleaning this muffler in an ultrasound cleaner with Simple Green
and I think that it will work. This is not a sintered kind of muffler,
so I have some reason to hope that the cleaner will do it.

i