Just use higher RPM 10 if necessary. More is good, and i don't think any significant amount diffirence there will be.
Most likely when you want it to go slower, it just lowers DC output. So RPM also gets low. If you can go %50 with 3 RPM motor, with same conditions you can go to %30 with 5 RPM. If you want to put more powder and go lower speeds at same time then go 10 or 20 RPM.