The download process for the Yellowstone maps is slow because there are so many map images, but in most cases you do not need all the images downloaded for it to be a very functional experience when offline.
However if you’d like to download everything, we recommend that you plug in your device to the charger and let it run over night to get a complete download.
You can also interact with the map in those areas which you are most interested in and it will automatically build up the cache for usage during offline times.