No longer a theory for me. Figured out I could test this, with an empty keg, posts with the various springs under them, and a QD with a length of CO2 hose attached to it, filled with water.
The short spring on the left allows the hose to drain into the keg at what I'd call a fast speed. The spring on the right takes this down to "drops", 2 or 3 a second. Fast drops, but nowhere near the speed of the short spring.
So, I'll try finding more short springs, I've already emailed Bobby's store and one more as a backup plan. I could probably just cut the longer ones but I'd prefer them to have a flat ground surface where they hit the bottom of the poppet.
If folks have foaming issues, you might consider this as a cause. I might have to give my "picnic tap 2.0" another shot, giving it a smoother flow to start with might help.
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