This one's obviously a bug , the barrow wights in the first 4 bloody crypts haste their undead buddies, but slow doesnt work on them.
(time stop which isn't in the game because it's broken, is an extreme example)
Slow like haste is a transmutation that changes the nature of time for the affected.
It is not a mind affecting spell, it works on the physical. Nothing is immune to either affect.
Constructs, spirits etc anything that can be hasted.. can be slowed.
Can we get this one fixed? I never see anyone cast this spell, and now I know why, it's been nerfed into oblivion for no reason. (I think from day dot)