Feeling of friction on back wheel. Wheel nuts on too tight?
Torque on the axle nut won't cause this per se. Think of each axle nut pinching the dropout really hard between itself and the hub locknuts. On a typical hub, if the locknut were loose against the cone, and/or if the axle threads were poorly lubricated, then tightening the axle nut can cause the axle to rotate relative to the cones, ruining the bearing adjustment and adding a bunch of friction. Shimano internal hubs with their burly anti-rotation washers are normally immune to this though.

Front wheels generally always spin way more freely than rears. All the Shimano internal hubs are somewhat on the draggy side. If you're noticing it when pedaling, make sure the chain is lubricated.

Things that could cause it to be worse than normal: poor hub bearing adjustment, need of internal lubrication using the special Shimano oil bath, dropout alignment causing excess bearing friction/wear, bent axle, something physically rubbing against the hub/wheel/tire, and brake rub.