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Hose diameter mainly affects flow rate. Q=VA. Q is Flow Rate(cubic feet per second), V is velocity(feet per second) and A is cross section area(square feet) of hoses(1/2" dia vs 5/8" dia vs 3/4" dia hoses). When you write poor performance you are probably writing about pressure(psi Pounds Square Inch). Each irrigation head uses up some pressure and some flow rate. I would guess that you can not run three impacts or 2 oscillating heads because the pressure couldn't supply that number of heads. The distance of throw of the heads is a function of the pressure. To determine your spigots pressure, you go to a hardware or plumbing store and purchase an inexpensive pressure gauge(~$15) with a fitting that screws onto you spigot hose thread. Let us know your reading. Also if you can let us know the Brand, type and number of your sprinklers, we can give you some suggestions on improving your system. Aloha