Heroes of Might & Magic 3: troop speed formula?

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 is 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.

Your hero also may carry artifacts which increase creatures' speed:

• The Necklace of Swiftness increases combat speed of all creatures by 1
• The Ring of the Wayfarer increases combat speed of all creatures by 1
• The Cape of Velocity increases combat speed of all creatures by 2

This is in addition to the bonus granted by Sir Mullich that Kyralessa mentions.

Source

There is another possibility: heroes with creature specialties. These heroes increase speed of the units they specialize in by 1 point.

For example, Beholders and Evil Eyes get +1 speed when used by the Ajit hero. Similarly, most of the creatures have the corresponding hero who specializes in them, with some exceptions.

More info: http://heroes.thelazy.net/wiki/Hero_specialty#Creature_specialties