First of all, make sure your tire isn't rubbing because the wheel isn't straight in the frame. Make sure the brakes aren't rubbing on the rim and that when you squeeze then release them they return to a centered position. Either of these things would produce more pronounced dragged than some of the other possibilities.

Next, overshift the chain inward so that it falls off the small chainring then try turning the crank by hand. If it feels smooth and turns easily then the BB isn't the problem. If it is binding you'll know right away. Also spin each pedal to see of either is tight.

Another possibility is a chain that needs lubrication. Don't forget, every link pivots a little as it enters or leaves a tooth on the front or rear. Finally, just spin the rear wheel and see if it spins down real fast. It should go for a long time. The bearings should be loose enough that the valve stem's weight makes it want to rotate down to the bottom. While you're at it, check whether the RD pulleys are getting tight. Usually they can be removed, cleaned, oiled, and put back in, but on some RD's it's not so easy a task.

I'm not sure what you mean by overtightening the axle. Did you mean tightening the QR skewer or nuts which hold the wheel to the DO's? Or did you mean the actual bearing cone? It is possible to overtighten the bearing cone, but they are normally locked into place with a locking nut inside the DO. Unless they weren't adjusted properly they shouldn't move when you tighten the nuts which hold the wheel to the DOs. They are easy to adjust properly but you do need special thin wrenches called cone wrenches. Also be aware that bearing get a little tighter when the QR skewer or axlenuts are tightened, so if they are on the tight side before you install the wheel they could become too tight. But I wouldn't think that would be enough to notice unless they are too tight to begin with.

Finally, check your tire pressures. Low tires shouldn't cause so much drag that you can feel it until they become noticeably low and threaten to pinch the tube or bang the rim on the ground.