In descr_battle_map_movement_modifiers.txt you can modify the speed of units in different terrains. However, this modification will affect to ALL the units.

In case of individual units, you can choose between several speeds by selecting the desired skeleton: fs_slow_**, fs_**, fs_semi_fast_**, fs_fast_**
Not all the skeletons have all the possible speeds, but most of them have three different speeds.

In case of you want some speed "fine tunning", thus you need to change skeletons and animations in 3dsmax.