Start with the basics.
Check the air filter. Is it dirty?
Change the spark plug for a new one.
Have you cleaned the carburetor? Perhaps a little Fuel Injector/Carburetor cleaner dumped into the fuel tank will open up a partially blocked jet.
Have you checked the throttle cable? Sometimes these can stretch causing the throttle slide or throttle plate to not fully open.
Is the chain in good condition? A neglected chain can rob horsepower that used to get to the ground.
Are the valves adjusted correctly? If they are too loose it delays the valve timing. If they are too tight the valves may be releasing compression and combustion gasses.
As was suggested, check the compression. After getting a consistent reading, put a small amount of oil into the cylinder thru the spark plug hole. Then run the test again.
If the compression is low and adding oil doesn't seem to fix it it indicates the valves are leaking.
If the compression is low and adding oil does seem to raise it, it indicates the piston rings are worn.
If the compression is high and the oil doesn't change it it indicates that all is well and you should be happy.