Looseness of the Unloader valve is not an issue as you say that dead ending the auxiliary valve has no effect on the slow lift!!
I think the issue lies elsewhere but ideally need the pressure test!!
Dead ending the aux valve during lift, rules out all problems in the Unloader and flow control valve assy if there is no change to rate of lift ??
While you have unit on the bench remove the union and check the flow control piston and its spring?? This piston (only) is matched to the valve body and could give slow lift, but if it was responsible for your issue the lift would speed up when the aux valve was dead ended!!