When the mower is turned off and the blades disengaged can you move the blades easy by hand? Are the blades sharp? When you cleaned the air filter did check to see if you could see light through the element. Hold the filter and shine a light on the other side and see if you can see any light, if you can then the filter is good, if no light the filter needs replacing. When is the last time you changed the oil and filter if it has one? What about the plug, is it clean or black? If its black your engine is running too rich.