Excerpt from the Manual:
When traveling overland, the movement speed (the distance a
hero and their army can travel in a day) depends on several
factors.
• Terrain type: for example, you can travel much faster on paved roads.
Where there are no paved roads, the movement speed depends on
the terrain itself and the hero's origin. If the terrain is "foreign"
to the hero, it will take them longer to cross it.
• Army: the hero's speed is determined by the speed of the slowest
creature in their army.
The hero will simply be able to travel a little less distance, but I don't agree that this will significantly hinder the elementals.