From what I now know....USB outlets on a PC charge at varying rates, depending on whether they're USB 1, 2, or 3.
USB 1 and 2 typically supply at 500mA, USB supplies at up to 900mA.
The dedicated Garmin ( or similar) charger with USB outlet will charge at up to 1500mA - hence much quicker !
Having a similar experience with a NAV 5 I did some research, and ended up buying a proprietary adapter plug to convert my charging cable from micro back up to mini size, so that I could use my old Zumo Garmin dedicated charger.
Hey presto - it works. Full charge in a few hours, just as you'd expect.