Slowing it down wouldn't do what you think it would do. I'm not sure what you mean by "prep work," but running it at a significantly lower speed would make it very hard to handle. The solution is probably to either use a finer-grit abrasive, or a different type of sander, or simply practice.

What jon said.

Hand held Belt sanders are a rough work tool made for taking a lot of wood in a short time.

My choice of prep sander---for sanding nost woods before staining/finishing is a random orbit sander with 150 grit paper.

If I need to finish sand exotic woods or between applications of some finishes, I use a 1/4 sheet finish sander with 220 grit paper.

I slow down my belt sander by turning the dial; it's a variable speed 4x24 Porter Cable. The lower speed is nice for finer control, especially while using it for scribing.
You should be able to use a router speed control so long as the amperage rating is compatible with the sander.
Casey