I'm assuming you have a BB with cartridge bearings, and not loose balls. I've had one that was very hard to turn with the fingers - like something was wrong. I reinstalled it a few times, trying various tricks, like tightening each side a little instead of completely installing one side. Somehow, I finally got it so that it turned smooth, with a little stiffness. I've lived with it for 1.5 years and just remembered about it after reading your post.
Bottom line, if the action is smooth (no gritty feel) but a bit stiff to turn, I think that's the nature of cartridge bearings.
In a way, I prefer the old school BBs because of their infinite adjustability and serviceability.