My Kawa suddenly drives slower than usual why???

Hello, dear forum visitors I have the problem that my bike(Kawa zx636r 9.000km run all inspections were made. ) no longer reached its highest speed, 1 week ago I drove 260 jetz driving the only 200. What can this be for help!!! and my bike is full at the zitern so at the tank I notice the full. besides that comes from my tank a whistle, I think that my tank leak is and that I need a new tank cover, Is the correct kt Please answer my problem. Thank you very much 🙂