Bret, I am a scroll saw instructor at the local Woodcraft store. I teach my students to start at a medium speed. On the DeWalt saw, that is 4. If you feel your cutting is going to slow, speed it up until you are comfortable. If you are having trouble staying on the line, slow the speed down. It is a comfort thing.
One thought though. When using reverse tooth blades, at very slow speeds, if you are not careful, the reverse teeth tend to lift the wood off the table. So you have to work harder to prevent that.