Its Renewal mechanics where you won't have instant cast regardless of your Instant Cast/Dex conf.
Find in src/map/config/renewal.h
/**
* Renewal Cast Time
* @values: 1 (enabled) or 0 (disabled)
* 1 : Cast Time is decreased by DEX*2+INT, 20% of the cast time is not reduced by stats,
* - for example, on a skill whose cast time is 10s, only 8s may be reduced. other 2s are
* - part of a "fixed cast time" that is only reduced by special items and skills (such as
* - Arch Bishop's Sacrament skill).
* 0 : the old cast time method, influenced by dex, items and skills.
**/
#define RECASTING 1
Change to
GHul's post in Instant cast -.-? was marked as the answer