I am trying to program a dual elevator on my 9C but am having problems with the controls. When I move the sticks slowly, the servos travel at the same speed. When I move them quickly the slave servo moves much slower. I swapped out the servos and tried a different reciever, but neither fixed the problem.

I think you need to change the channel assignment of the slave - you don't say if you're using PCM or PPM, but you do need them on adjacent channels if PPM or same channel group if PCM.

What you are seeing is normal. The two elevator servos are on different channels and work together because of a mix. The channels are quite a ways apart in the counting order, so one channel gets commands slightly after the other one. This can be noticed if you move the sticks rapidly. Please note that you can move a stick much faster than any servo can travel, so even if you had one servo, it will lag behind the stick.