What could be causing excessive friction in a new rear hub?
Adjusting the bearing is quite easy although it's more than just unscrewing the 2.5mm hex-screw at the left side of the hub. Here's the link to an instruction sheet from Campagnolo.

https://www.campagnolo.com/media/files/035_68_Technical%20manual_wheels_adjustment_hub%20Campagnolo_12-12.pdf

While the PDF describes how to remove excessive play easing an axle that is too sticky goes the other way round with one small addition.

The steps are the following: 

Untighten but don't remove the 2.5mm hex-screw that pinches the left cup.

Hold the right-side axle steady and turn the left anti-clockwise (to untension the load on the bearings) by 1/8 turn.

Check if the axle turns as desired. If it's still too tight repeat the previous step.

--While it's easy to tighten a bearing that has too much play, it may be tricky to loosen one that is too tight. A slight whack  on the right axle stub with a rubber mallet is helpful. (Speaking from experience with Campa wheels!)

Check if you didn't put in excessive play.

Retighten the 2.5mm hex with a torque wrench @ 2.5Nm.

But as I suggested in a previous comment I'd take the bike to the dealer because the wheels are rather new and still under warranty. And do it quickly, an overtightened bearing is easily destroyed.

One more thing: new rear wheels may be a little bit 'stickier' than one might think good. It is often due to the fact that they have to bear more load and resist to the pull from the chain. A couple of hundreds of km is often required to have them running properly.

They do have a video showing the overhaul sequence:
https://www.campagnolo.com/WW/en/Support/complete_revision_of_campagnolo_wheels_hub