The bike is an Avanti Discovery 8 which is a hybrid with a 8 speed Shimano in-hub gearing system.
The problem is I feel like there is definite friction on the back wheel. Spinning it freely it just doesn't travel as far as the front wheel and pedaling feels slightly impeded.
Obviously, brakes are easy to check and they're fine. There is no noise whilst coasting either, so I don't think it's the bearings.
I was wondering could it be something to do with the wheel nuts being too tight? I had a problem recently where the rear wheel was moving forward slightly causing slackness in the chain. I'd fix it, but it would keep happening so one day I really tightened it as hard as I could (the slack chain had come off causing me to crash).
Could this somehow be putting too much pressure on the hub?