Those flows assume copper pipe...pex has a smaller ID so the curve would be flatter and more horizontal. The pex could be part of the problem. 3/4" pex is about the same ID as 1/2" copper, so 1/2" pex is closer to 3/8" copper. If you can access things, and switched the pex to 3/4", you might notice a difference when running both. I don't think you'd need to change the valve to a 3/4". Actually, if you made the connections after the valve with copper, it would probably be best.