Yes, when in the SI view hit MENU once for the Express Menu to popup and it will be near the bottom. Scale is 1-10 with 1 being the slowest.
However, for the best images, you will need to match your scroll/chart speed to your speed you are scanning at (or at least keep it within 1 click either way. So for 3mph, a chart speed of 2-4 should work. I would tend to keep on the higher side at 3-4.
Good Luck and post us some snaqpshots so we can see some of your scans and whats going on.
Robert