Refer to figure.
Performance factor is obtained from the aircraft performance monitoring program every airplane from an operator’s given fleet is subject to. This index can be found on the operational flight plan provided to the flight crew before each flight so that it is crosschecked and corrected if it differs from the FMS. In fact, this factor is very important because it is used during the flight planning stage of the flight and it is a reference for the FMGS to maximize fuel economy on each and every sector. The performance factor will ‘guide’ the FMS to provide the optimum speeds, profiles, cruise levels, for better fuel predictions and more accurate managed operation information. In order to increase the fuel savings, the performance factor will help to correct the predicted fuel flow when the actual aircraft drag, and engine performance differs from the nominal model due to the aircraft’s age.

On the Airbus A320, you can find the performance factor on the ‘A/C STATUS’ page after selecting the DATA selector key on the MCDU. The performance factor is situated right next to the idle factor and can either be positive or negative.