Slows Target by % reduces target attack rate and velocity by the displayed percentage when applied by a successful attack (although Clay Golem also applies it to attacker when struck by a melee attack).
Percentages from multiple items equipped by the same player or hireling are summed, although when two weapons are equipped only that of the attacking weapon is added (that of the other weapon is ignored).
Applied by any melee or ranged attack with the exception of Blade Sentinel, Blade Shield and the additional arrows of Multiple Shot (only applied by the original two arrows).
Maximum ST 90% applies to normal and minion monsters, and pets: if ST 100% is applied, 10% is always redundant. [1]
Maximum ST 50% applies to player, mercenary, Champion, Unique, Super Unique and boss monsters: if ST 100% is applied, 50% is always redundant. [2]
Sums with attack rate and velocity reduction from cold length, Holy Freeze and Decrepify, although no unit can be reduced below 15% attack rate or 25% velocity. [3]
Slows target for ~30 seconds (not reset when target makes successful attack). ST from separate units does not sum: first percentage applies to velocity (subsequent applications before expiry will reset duration for which this first percentage is applied, even if different, until last applied expires), while most recent applies to actual attack rate. Different percentages applied by separate units or attacks may result in attack rate desynchronisation: while the most recent percentage is always applied server-side, the first continues to be applied client-side until last applied expires. [4]

Attack rate
15 frame AI delay in all difficulty levels lengthens the interval between consecutive attacks.
Weapon speed and Increased Attack Speed from armor or weapon affect attack rate.

EIAS Frames
-20 20
-19 19
-15 18
-10 17
-5 16
0 15
8 14
15 13
24 12
34 11
46 10
61 9