first off check the linkage on the auxillary functions on the control valve and make sure they are moving in full motion in both directions.
secondly, check the sideshifter cylinder(s) and see if they have any additional flow control valves mounted at the cylinder line fittings, sometimes they use adjustable valves to control the speed of the sideshift so it will not move too suddenly or too fast (for stablilty purposes).
If there are no adjustable valves on the cylinder fittings on the cylinders then go back to the control valve, there are probably some orfice valves located in the middle of each section that can be removed and changed. Some models these flow control valves consisted of a removable pin that is inserted into a chamber that closes off the flow through the chamber, these pins can be ground off some to allow for more flow or changed out to longer pins to decrease flow depending on what you are trying to accomplish.

If you do not find either of these scenario's above then it may be something wrong inside the control valve itself. But generally if your direction speed is good in one direction and not in the other you would be looking at the lever actuation of the spools in the control valve sections or something at the cylinders themselves.
You can try to reverse the lines on each function and see if the problem reverses itself, if it does then the problem is in the control valve, if it does not move then you will be looking for a problem in the cylinders.

Now there is one other less common scenario where one of the hydraulic lines/hoses may have collapsed inside restricting the flow of fluid in that direction so you might want to remove the lines/hoses and check them and make sure there is nothing blocking the flow of fluid through them.