That's not possible, quite simply. You must have made some other change at the same time, so you are relating the issue to the Addon Manager.
The installer or the Addon Manager won't change your FSX.CFG, unless you told so, by changing some of the sliders and pressing the "Save to FSX" button. Until you press the "Save to FSX", nothing is saved in your FSX.CFG, which is the only thing that might potentially change your frame rate.