As stated - as soon as you're on the approach radar freq.
I would define slow speed where the crew are unable to fly the standard speeds - ie 180 base leg/ 12-8 dme and 160 to 4 dme.
To announce this as the turn onto the loc is issued is too late and I would break that aircraft off as the sequence has been set up at this point.
I agree that the controller should've used speed control - but then maybe he did and the crew decided that that didn't really apply
in which case I would have broken that one off !
louby