This is a common issue with shim and bucket valve trains. As the valves are getting worn into the head a small groove is beat into the valve face after opening and closing so many times. As the valve pushes up into the head it takes up clearance between the cam lobe and the bucket which the cam lobe depresses to push the valve in and open it to either release exhaust gasses or allow intake mixture into the the combustion chamber.
As the valve gap is taken up over time a new shim that is thinner needs to be inserted under the bucket. This is a normal maintenance event to add clearance between the cam and the bucket. If this is not done, a gap between the valve face and the head can form, typically this occurs on the exhaust side (don't ask me why) and the gap creates an exhaust leak, essentially allowing exhaust gasses to escape during the combustion process. In the beginning stages of this, fuel mileage decreases as does performance. In advanced stages the bike will not start or it will run extremely poorly.
Adjust the valve clearance to fit within service limits.