First, isolate the problem to see if there is an issue with the servos speed, or a programming issue in the radio. Just swap the two servo plugs. If the same servo is slower than the other, its an issue with the programming of the servo. If the problem moves to the other servo, then there is probably a programming issue in the radio. On the DX8, each channel can have the servo speed slowed down. Gotta be one or the other.
Good luck,
Pete