Used a V1 and a Garmin NAVI unit both plugged into the cigarette lighter socket in the dash of my Boxster and these devices never appeared to interfere with anything.
Concurrent with, or shortly after the cabin vent fan speed drop did the cabin temperature feel like it went up? Did the cabin air feel humid?
If so that's a sign the A/C system for some reason had developed what is in my experience totally out of character some kind of intermittent inability to keep the cabin at the 72F temperature.
If not, then it is sort of normal behavior in my experience.
In auto mode with the temperature set to 72F after a while the cabin vent fan speed drops to below the half way mark. The A/C system is very effective at cooling the cabin and once cool doesn't have to work too hard.
Sort of normal behavior... I have to mention that in my experience with my Boxster (and Turbo) the fan speed in auto climate mode (or in any mode) never makes sudden/big drops in speed, although under certain conditions like when starting out with a hot car and cabin as A/C system gets the cabin temperature under control the cabin vent fan speed will come down from high to about half speed over just a few minutes.
On the passenger side of the dash there is a small round grill. There is as least what I understand a small fan there which is used to circulate cabin air over a temperature sensor. This is to let the climate control know the cabin temperature and if the cabin temperature varies from the desired temperature the climate control system will adjust various things -- air flow doors, fan speed, A/C compressor displacement -- to bring the temperature back to the desired setting.
If something blocks this grill this can affect the auto climate control's ability to keep the cabin temperature at the desired setting. The system can react rather quickly.
Or if something was causing cool air from say the dash vent on the side of dash towards the grill opening this could cause the A/C system to believe the cabin air temperature was lower than it actually was and cause it to cut back on the air flow. Thus a drop in the cabin vent fan speed.
Never had to have any A/C work done but my info is the auto climate control system is pretty sophisticated and has some self diagnosis capabilities. If you are unable to find anything to explain the behavior a Porsche tech with a Porsche diagnostic computer can query the auto climate control system's controller for any error codes and from this be able to diagnose what's wrong, if there is anything wrong.