Have you checked there is plenty of oil returning to the the oil tank?
A proportion of the oil returned to the oil tank is bypassed to the rockers, by the action of a slight restriction placed in the return where it attaches to the oil tank, the restriction will hardly be affecting he return rate, as if needs be, the return side of the pump could reach the same psi as the feed side, in practice it doesn't, as there is very little restricting the oil flow back to the tank.
A rocker feed copper pipe with too small an I.d. Might slow the oil supply down?