windshields arent quite as bad as you might think for drag. remember, if it wasnt there it would be your upright body dragging the wind. The screen actually gives some assistance on an upright riding position.The carb is clean and has been rejetted. No rust in the tank. Nothing indicates issues with the engine. I also have a windshield, so that adds resistance.
But I must repeat, your bike is not right. Go back to basics and check all the jet sizes and needle type and position. something somewhere is wrong. Maybe valves are too tight? 800;s are weird in that almost always the valve adjustment gets tighter with wear, rather than looser. so the quieter the valves are, the more you should check them.