Ninjutsu is affected by Fast Cast. With Loquacious Earring + Haste, you gain 1 second recast on Ni. And just see with RDM/NIN, you can reach a 20ish Ni recast thanks to your Job Trait + AF2 Body + AF1 Hat
Bought with 75 Ancient Beastcoins from Sagheera in port Jeuno. Must have access to "sea", meaning you have to be past Promathia mission 7-5 in order to make purchases from Sagheera.