I never had problems with large overland maps, only with large city maps. Please read page 22 of the CC3 Essentials for your scale question. Scrolling is not a good choice to move on your map: click "zoom extents" and then "zoom window" to move to the desired area, or configure the mouse under tools->options to move around the map after pressing the mouse wheel. Also adjust the display speed settings as long as you work on the map and of course, turn off the effects.
Good luck for your project