The reducer - coupler - may have been the "restrictor". Anytime you reduce the diameter of a pipe, you speed up the fluid flowing through it and reduce the pressure that the fluid exerts on the wall of the pipe. Going from a 3/4" to a 5/8" hose is about a 30% reduction in cross sectional area (about 0.44sq.in to 0.31 sq.in.). That seems like a relatively significant decrease. the inside of the reducer is probably beveled to give a smoother transition.
I don't know if the reducer is standard/original, but it seems logical that it would be since the pump outlet is obviously 3/4". How did you cram a 5/8" hose onto a 3/4" nipple anyway??
Tom S.