I never bother to use each of my GPS units on a frequent basis. Some of mine have not been used for a couple years. But, I have always found that if I turn it on outside where there is a clear view of the sky for a 15 to 20 minutes, that way the GPS can download the revised satellite orbit information.

When you first turn on a GPS, it looks at the internal clock and looks at the orbit data in its database and tries to figure out where the satellites should be. It then tries to get a location fix based on that data and it starts assuming that your location has not changed much since it was last used. Thus, if the internal clock is wrong or if the orbit data is old or if you have moved the GPS unit a few thousand miles from last time it was used, each of those things can cause it to start up much slower and with poor accuracy.

I am not disagreeing with the Garmin tech you talked to, the Garmin people built a business on GPS units and they know their stuff. (I used to own stock in Garmin a decade and a half ago so I have an idea on the company history.) I am just elaborating more on why it can start up kind of slow.