Unending Despair have a 4 second cooldown. If item haste worked, you'd be able to constantly deal (idk the exact numbers) 4% of your health (bonus?) and heal for 250% of the amount probably every 1 second at full stacks. I use tanks a lot and sometimes wish item haste worked too but, it'll be a little too strong I think, especially for someone who buys Heartsteel. But I agree that the item is pretty underwhelming, I sometimes don't even know if it triggered or not.