Fast Cast goes through when the Spell begins casting (so equip Before).
Fast Cast's
timer reduction effect kicks in at the end (so keep this on at the end if you're looking for a lower timer).
Skill+/MAcc kicks in at the end, so put it on at any point before it casts.
MAB+/INT+/MND+/CHR+ kicks in at the end, so put it on at any point before it casts.
Example:
BLM casting Thunder IV.
Start with Loquacious Earring & Warlock's Mantle (if you really want the 4% Fast Cast effect >_>) then swap out to Rainbow/Prism/Ixion/Whatever cape and INT+/MAB+ earring when you're casting (before your Thunder IV goes off).
WHM casting Cure V.
Start with Cure Clogs on then swap out to Errant/Mahatma/Blessed feet in the middle (slightly more annoying because of the cast time of Cure 5) to get maximum effect.
RDM casting Utsusemi.
Start in AF Hat, AF2 Body, Loquacious Earring and keep them on for the
full cast to get -22%(?) Casting time and -11% Recast Time.
RDM casting Blizzard III.
Start in AF Hat, AF2 Body, Loquacious Earring and swap them out for Errant Body (or whatever your nuking piece is) and INT+/MAB+ earring.
You won't get the "Haste" effect of Fast Cast due to them not being on, but you
will cast sooner because they were on at the start.
Items such as Yigit Gomlek, that have Song Spellcasting time- will (shockingly) only reduce the Casting Time of BRD Songs and
won't affect the recast timer of them.
I've prolly skated right around the question, let me know if I have and I'll try to be more useful XD