Dodge, I understand about WB barb connection sizes.
What I am saying is that by replacing the 1/2" with 3/8", you are lengthening the restriction caused by using 3/8" ID from just the barb to the whole tube.
Restrictions aren't 'all or nothing'. The resistance in the loop is not the same as the highest resistance in the loop - it is the sum of all the resistances in the loop.
By reducing the flow diameter from 1/2" to 3/8" you force the water to increase velocity to pass through the restriction. This will cause a pressure loss (as well as increased flow resistance due to the smaller ID and faster fluid flow). However, if you run the whole loop at 3/8", you will have the increased flow resistance from the smaller ID and higher velocity through the whole loop, not just in the barb(s).
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