Do you realize that each AI turn would take minutes if the game used perfect pathing for every freaking unit it moves (and it generally moves almost every unit once per turn at the least)?

nikolai: It could be possible that the game uses a kind of large-scale pathfinding via road join points (where roads go over the region border). This way, the AI stacks would usually travel reasonably fast because travelling on roads is faster than through wild terrain and it could be pre-calculated quite easily.
If that's true, you should be able to stall the AI by putting a large stack between their target and them onto a road join.
Keep up the good research