I suspect your flow problem is due to the 1/2" CPVC. The interior diameter of CPVC is approximately one size smaller than copper pipe of the same size. I realize this sounds weird, but what I mean is 1/2" CPVC is approximately the same inside as 3/8" copper. 3/4" CPVC is about equal to 1/2" copper. I think it is this size difference is what you need to compensate for.