there are multiple negative velocity modifiers
cold damage provides 50% slow to both attack and movement speed. This is prevented by cannot be frozen modifiers.
slows target provides the single sum of item sources or clay golem up to 50% slows target to players, mercenaries, champions, uniques and super uniques- to both attack and movement speed. It provides up to 90% slow against normal monsters and minion pets.
decrepify provides 50% slow to both attack and movement speed
holy freeze provides its listed value as both attack and movement speed slow, capped at the targets chill effectiveness parameter- players, pets & mercenaries cap at -50%
attack and movement speed slows are directly summed together against +% velocity modifiers for attack and movement speed like fanaticism, vigor, %EIAS calculated from %IAS on items, etc
the final movement/attack speed has a negative minimum cap, -85% for attack speed and -75% for velocity (reducing from normal 100% velocity to 15% / 25% velocity)
cannot be frozen modifiers have no effect on slows target, decrep or holy freeze.
In PvM you generally reach the maximum slow rate from any 2 skills combined, you'll only be partially offset against fanat mobs or extra fast or whatever, but its redundant to use many slows against other targets
in PvP players tend to have more %IAS and %FRW to offset against it. A character with +120% FRW has +66% velocity, so it would take 141% combined movespeed slow to bring him down to the minimum movespeed speed