Compared to the 3rd-level spell Haste, Improved Haste target's a single creature, doubles it's attack rate, and does not cause fatigue when the spell duration ends.
|“|| Improved Haste|
The affected creature functions at double its normal movement and attack rates and gains a - 2 initiative bonus. Thus, a creature moving at 6 and attacking once per round would move at 12 and attack twice per round. Spellcasting and spell effects are not sped up. There are three major differences between the 3rd-level wizard spell Haste and this spell: First, there is no period of fatigue following the expiration of the spell; second, this spell will only affect one creature; third, this spell actually doubles the number of attacks per round of the hasted creature. This spell also negates the effects of a Slow spell, but it is not cumulative with itself or with other similar magic. Its effects will last for the duration or until dispelled.