could well be hoses Stu.
look for...........
-too many bends/fittings in a hose run
-wrong type of fittings may look right but have a smaller bore
-check where the flow goes back into the pump. If it enters back through a built in valve that may be designed to restrict one way (remember those pumps are basically the same as industrial units)
-simply there may be a slow down valve ?
pics of the install ??