Pathfinding focuses on the shortest distance as you illustrated. The scene editor allows movement costs to be added to terrain navmesh faces, but TW have not used this on their worldmap (mainly used by them in mission scenes to make certain routes less preferred). I presume it isn't used on the campaign map navmesh as TW have coded varying movement speed through forests for different factions. Movement penalties can be modded into the campaign map as additional navmesh movement costs, but I don't know how much of a performance impact this may have on low end pcs.
The movement costs added are in game metres. The shortest route algorithm just factors those additional metres into its distance calculation.
