Rear Wheel Drag / Friction?

I was cleaning up both mine and my nephew's bikes yesterday when I noticed that his seems to have some sort of issue which causes the rear wheel to stop pretty quickly after you spin it. The best example of this I can give is that if I spin mine and his at the same time, and with roughly the same force, his wheel stops spinning after about 7 seconds, and mine after about 30 seconds. You can even feel it when just pushing the bike along. His feels sluggish.
Is there anything I should look at to try and solve the problem? He's quite new to mountain biking and struggling badly with hills so I'd hate to think he's working harder than he needs to be. I've checked the brakes and there's no rubbing, and there doesn't seem to be any contact anywhere else either. There's no mud or grit that I can see.
Mine has a different hub to his, some old Shimano Parallax thing on mine vs an All Terra hub on his.
Any ideas?
Is there anything I should look at to try and solve the problem? He's quite new to mountain biking and struggling badly with hills so I'd hate to think he's working harder than he needs to be. I've checked the brakes and there's no rubbing, and there doesn't seem to be any contact anywhere else either. There's no mud or grit that I can see.
Mine has a different hub to his, some old Shimano Parallax thing on mine vs an All Terra hub on his.
Any ideas?
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+ some other bikes.
Oddly enough, I put it back on and it seems much better. I guess there must have been some grit somewhere that I couldn't see because it's much, much better now. Oh well, one of those things I guess. Thanks for the advice.
+ some other bikes.