Under NO, NO, NO, circumstances do you want to wax your hull. Sudden loss of speed could be a number of things. 1st thing, get your tach fixed, hard to identify the problem until we know where the motor is running. 2nd have you added a jackplate or messed with the throttle linkage any? Anyway your throttle linkage isn't opening all the way up? 3rd, plugs and wires, you said you replaced but did you replace with the right plugs? 4th, has you prop gotten ragged out around the edges? A good finished balanced prop can make 4-6 mph top end difference. 5th, compression let down test, may need an overhaul.