Cast Time is composed of "Variable Cast Time" (VCT) and "Fixed Cast Time" (FCT). As the names may imply, VCT depends on the stats and gear of the caster, while FCT is constant for a given skill. A small number of effects can reduce FCT. Effects which reduce FCT by a percentage do not stack; effects which reduce FCT by a set amount do stack, and are applied before percentage modifiers. For most skills, FCT is 20% of the total Cast Time, with the remainder being VCT.
VCT is reduced by DEX, and also INT at half the amount.
Cast Time Formula is RENEWAL!
Related to Renewal feature and also implemented by Ragnarok Online MSP, every skills have Fixed Cast Time that never can be reduced by stats (DEX or INT) but they set Fixed Cast Time to 0 for all skills.
Calculation
Cast Time (seconds) = VCT + FCT
VCT (seconds) = BaseVCT x (1 - SQRT[{DEX x 2 + INT} / 530]) x (1 - Sum_VCTReduc / 100)
FCT (seconds) = (BaseFCT - Sum_FCT) x (1 - Max_FCTReduc / 100)Ever wondering why some skills in Ragnarok Online MSP has no cast time like Sage Hindsight, In Renewal there is a 3 sec Fixed Cast Time but in Ragnarok Online MSP there was no cast time even you don't have dex this is simply because they remove the Fixed Cast Time (not totally remove, they just simply 0 all Fixed Cast Time for all skills)
The Variable Cast Time is now 100% instead of 80%, so the Overall Casting Time of Skill can reduce by DEX and INT Combination!
If you are a Wizard and have a 90 Dex and 90 Int you can achieved a Fast Cast like Pre-Renewal and can achieve Instant Cast if you have a total of 530 combination of DEX*2 and INT (for now is imposible).
Casting Lv10 Meteor Storm in Renewal has (5.148 seconds (VCT 2.748s + FCT 2.4s))
In Ragnarok MSP Casting Lv10 Meteor Storm has (2.748s seconds (VCT 2.748s + FCT 0s))
Conclusion
Do not believe on someone saying that you should go to DEX 60 - 70 because there's no difference to 90 DEX! that is big mistake! 90 DEX has a big difference in casting time.