Looks like in your profile pic you have stock mirrors, but since it is somewhat relevant I'll point out that bar end mirrors have a way of rubbing grips and adding friction to the throttle grip, especially when the rubber isn't cut squarely.
While you don't want to lube the cable, you DO want to lube the cable ends, both at the grip and at both carburetors.
You can perform a simple test to see if the throttle grip is causing the problem. Disconnect the cable up at the handlebars and pull on it. If the action is smooth, the suspect is at the top end. If not, you can disconnect the lower end and check the action at the carburetors. If they are smooth, it is most likely a worn cable.