If you haste your units before a fight the human player will have to slow them twice to be able to slow them once to remove haste and a second time to slow them...
you need only one, haste/ slow wont stack, both spell will remove if u use another slow->faster faster->slow
if u play faster orc player will put A click lust then he will go to his base and control something else, new exp/rax making more ogres
while human will sill micro
haste dk> invis mage
you can block invis mage while hasted dk will kill your all units if u try block him, ofc buldings will block haste and invis