If you COASTING through this rpm range while slowing, there is no way this is "lugging", however if you are starting to accelerate again, it depends on how much throttle you are adding. ASSUMING you are coasting, I would suspect a mild misfire. You have a VERY LOW mileage bike for that age and I would suspect varnish in the fuel system from old gas at some point in its history. A fuel system/injector cleaner is an inexpensive experiment along with checking spark plug condition. Caution: cans of fuel system cleaner are sized for about 16 gallons of fuel in a car gas tank, you have 6 gallons, do the math and mix it in the proper proportion or you could do more damage than good. One can of cleaner would treat about three full tanks. If that doesn't help, you have a different problem. IF you are trying to accelerate back to the speed limit through this rpm range, you are lugging and that is your problem.