Slow effect

Slow is a status effect that doubles the duration of a round only for the creature it is applied to, unless they are explicitly immune to it.
Double round duration means that:
*Movement speed is halved
*Attack animation speed is halved
*Casting time is doubled
*Spells can be cast only half as often (eg if a Mage is slowed, they'll still be able to cast a spell in the same round they were affected, but not on the next; and it alternates afterward)
*Items can be used only half as often (works just like the effect above)
*Using an item takes twice as much time
*Attacks per round are halved, despite the UI not reflecting that (characters with only one attack per round will attack only every other round)
*Initiative roll suffers an unknown penalty
*Stealth's cooldown is doubled
*Regeneration is halved
*The creature basically takes twice as much time as it should for all tasks
==Gameplay==
*Slow and Haste effects cancel each other out. They don't remove each other, but all of their effects are neutered until one of them wears out.
*Slow does not alter a creature's AC or THAC0, these are separate effects applied by the Slow spell in addition to the Slow effect.
*Certain types of Disease can slow a creature exactly like the Slow effect, but they are still Disease effects: Immunity to Disease will work against them and immunity to Slow will not. They do however share the same interaction with Haste effects.
Since nothing cures Slow per se, it is advisable to consider the source that applied this magic. More often than not, it is dispellable magic.
Immunity to slow can be achieved with various items, but here again it might be worth considering the source trying to apply the magic instead. Maybe it's a spell or a weapon that can be made ineffective.