Lost performance is almost all due to two factors - engine power degradation and rotor blade erosion.
The engine can be checked by using a routine hit/power assurance check, and comparing the engine to delivered norms. Power assurance margin is directly usable as a tracking tool.
rotor blade erosion is harder to track, since there is no easy way to find the lift vbs drag characteristics of a specific set of blades. One way is to conduct a climb power check, using a well calibrated torque gage and a known gross weight to compare the climb performance to that called out in the flight manual.
Wear on controls, increased door gaps and such will not cause any appreciable loss of power performance although control slop can cause vibrations.
Aircraft (and pilot!) weight gain is easy to find, just use a scale. Most aircraft gain about 1/2% empty weight per year due to equipment increases, spilled oils, dirt and paint.