I'd suggest slow especially if you find yourself speeding up the corn varible to get head going fast enough for ground speed. I have heard some talk about leaving on fast to eliminate the need for stripper in front of rear feed sprockets. I'd hope someone more familiar with corn will help you out hear. Size of head, average yeild, and maybe a couple other things likely enter in hear. I think there are different sprockets for conveyer speed also and don't know just how to determine what a guy needs for various yeilds and conditions. I do know I do like the slope kit to help material make the transition between header auger and feeder. I also had to extend flighting in center area on all our heads so flighting intersects a center line (otherwise material stalled and blocked flow coming from one of the rows). Good luck