More testing: I stuck the maps (2015.40) on an SD card and put that in the slot behind the screen to see if it helped at all. When the nav boots up, it does say "loading maps." during the nav boot sequence, and it doesn't do that when it loaded off it's internal card. I waited until it was fully booted up before putting it in reverse and pulling out of the garage.

The map was ok until I started driving down the street, then it froze and the stuttering began. Backing out to the main nav screen (where do you want to go or map view) and clicking on the gps meter in the upper left showed that I could see a number of satellites, and that screen updated just fine. Trying to go back to the map hung with the Map view icon highlighted. So the lag seems specific to the map render. The system has the new 2.9 nav software, 2.70 American Jill voice, 2015.40 North america detail map, and 2.0 basemap.

USB music playback is fluid until the map hung, and then the stuttering began.

During all of this, I'm able to get back to the menu, or the source by clicking on the source secondary bar from nav. Getting back to nav by doing the same (from the source window) will lag badly or be ignored.