Have you recently switched the enhanced graphics on or off?
The enhanced graphics do cause some devices to work harder than others. I suggest switching the enhanced graphics off to see if that resolves your issue.
You do this from the menu in the lower right corner of the game, then click the settings gear, and slide the "enhanced graphics" toggle.
To group armies you can also use the "add army" button and then select the "+" button under the other groups you wish to select. Once you have them all selected click the checkmark button and then give one order to move them to a single location.
You can also use the delay feature so that they all arrive at that one location simultaneously.
