The stat decrease that can be applied by Chill Spd is considered a penalty.
Any 【Penalty】 on a unit, including penalties, lasts until that unit takes an action. If the unit takes multiple actions in a turn, they will only have the penalty for their first action.
Chill Spd does not compare Spd stats.
Because no comparison is made, skills like Phantom Spd 3 have no effect.
Unit selection takes place after all bonuses on allies are removed, and before any new bonuses or penalties are applied. All other existing bonuses and penalties count towards their respective stats during unit selection.
If multiple units are tied for the highest Spd, each of those units will receive the effect.
The Traditional Chinese name for this skill used to be 速度封印, a.k.a. Seal Speed, when the skill first came out. This was subsequently changed to the current translation in a minor update. (Japanese used 封じ instead for Seal passives.)
penalty.
Phantom Spd 3 have no effect.
bonuses on allies are removed, and before any new bonuses or 
Chill Spd 1: Create for 20 
Great Verdant Badge, 100 
Verdant Badge, 40 
Sacred Coin.
Chill Spd 2: Enhance for 60 
Chill Spd 3: Enhance for 400 
