From Project: Redcap
The name of the Virtue is a bit confusing. It has nothing to do with the "fast-casting" option for spontaneous spells (ArM5, p. 83). All the Virtue does is give you a +3 to Initiative rolls when you cast spells in combat.
Normally when you are casting spells in combat, you roll initiative like everyone else. Spells have no weapon initiative modifier (they're not weapons) so your spell's initiative bonus is just your Qik + Enc. If you have the Fast Caster Virtue, you add an extra +3 to that.
- Fast Caster Virtue, ArM5 page 42
- Magic in combat, ArM5 page 174