When dealing cold damage to an enemy, what all does it slow down? I know it obviously slows down an enemies movement speed, but does it affect casting time? Or does it slow down their attacks also?
Cold damage applies a snare effect to the enemy. This effect slows them generally, so yes, it slows their attacks, casting and so forth.
I have no citation for this other than my extensive use of cold weapons and caltrops over the last 60 levels.