Creature speed isn't modified by hero stats...with one exception. If you have the Armageddon's Blade expansion, you can hire Sir Mullich, a Castle hero, who gives +2 to all creatures under his command. See here: Armageddon's Blade features
Creature speed isis modified by the terrain type where the battle takes place. Creatures get a +1 bonus to speed if they're on native terrain. If I remember rightly, for the Fortress creatures, that's swamp; for the Tower, that's snow; and so forth.
Creature speed can also be modified by spells such as Haste and Slow, of course.