Why do you suggest a slower speed for polishing glass?

While you can polish at the same higher speeds that are good for grinding glass, you will run into a few caveats.

First off is the heat buildup. Polishing is accomplished by the friction between the cerium oxide and your glass. Polishing pads are made to provide that friction. Of course, friction generates heat and large heat buildups in targeted areas of a piece of glass can lead to all sorts of problems.

At faster speeds you generate more heat on your glass faster. Therefore you have to be more observant of the surface temperature on your piece. You can mitigate this by using more water to keep the piece cooler, but then you cut down on the friction required to get your piece polished and increase your polish time.

With the faster speed you will also find that most of your cerium ends up going down the drain. The centrifugal force of the wheel will sling most of your cerium off the wheel before it has adequate time to try and polish your piece. You then have to use more and more cerium to accompolish your polish.

Any time savings you think you might be gaining on the grinding end are eaten up by the longer polish times and the cost of more cerium to accomplish that polish.

If you have a single speed machine, set it for the slower speed for the disk size you have. You won't really notice a significant loss of grinding efficiency, but you will certainly notice an improvement in your polishing.