When one toon follows another and you move the first one, a message must be sent from his copy of WoW to the server before the second toon moves, and messages must come back from the server before either copy of WoW can display the second toon's new position. That's the least number of messages that a game can use for this purpose; WoW may use even more than that. These messages take time to go back and forth. There's no way to eliminate this lag because signals can't travel faster than light, but you may be able to reduce it by lowering your ping time. See this thread:

the latency trick

As for your observation about minimizing the window, my guess is that this is due to Windows's thread-scheduling algorithm. Windows gives more or less CPU time to programs depending on factors such as the one you mention. If you're curious, you can test this hypothesis by starting a utility that comes with Windows called Performance Monitor and seeing whether the second WoW's thread priorities jump up and down as you minimize it and restore it. In addition, WoW itself may allocate less time to moving the toon when its window is minimized.