Jim,
Right now they are stored on the "Device Storage" sometimes referred to a "USB Storage" on some phones. But not the SD card. Future versions will have the option to store the maps on the SD or Internal storage. Check in /storage/emulated/0/Android/data/com.matrangola.cuesheet.ui/files/maps in your S4. Sometimes the root name is different for other devices.
Yes it is a little slow but once it we have an area it will apply for multiple routes. Even those that overlap. For example, if you had downloaded a map for a route in the Northwest part of town then another map that included both the Northeast and Northwest part of town, you it won't re-download the Northwest tiles for the second map.
-Geoff