All Hi-Tec digital servos have the capability to have there speed programmed, did you verify the settings on a Hi-Tec programmer?
The Hi-Tec HPP-21 is a handy tool and it is relatively inexpensive.

Also (as a test) can you mix two separate channels together to verify the servo operation on independent channels to eliminate any possible influence that Y may have on the operation.

As mentioned, it could be as simple as the geometry of the linkages

Edit - I see you checked the linkages