I don't know what the problem was with the old wheel, but the hub is integral to the wheel. The wheel consists of a rim, attached to a hub by spokes. It's all one piece.
As for the difference in resistance, it might be that the gearing is different if they changed the sprocket cassette, or it could be that there's some break in period for the new hub.
OTOH, the biggest noticeable difference in resistance (in the same gear) comes from changes in tires and/or tire pressure. Your problem could simply be lower tire pressure. Or, and don't be offended, it could just be your imagination (this is common).
I suggest you go back to the shop, and have them confirm that the hub (and wheel) is OK, so you can enjoy some piece of mind. Going back is important because there might actually e somethig wrong that needs to be adjusted out.
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