Good luck finding a Fenrir card for 10-15b :v, i default temporal dex for that same reason on my WL, otherwise the cast time tends to be not as fast as someone wants even with 100% VCT reduction and i just slotted them with Nightmare Verit to keep the damage up. It's the best of it.
MVP and other cases i switch to Temp Int with Nightmare Verit or Temp Int Spell/RM
lol I am not gonna look for that, 10-15b is way out of my league, and I doubt its for sale even if the price is offered. It seems that the skill Radius also reduces FCT, it can be a total of near 37-38% reduction(15% base reduction + INT%/15 + LV%/15). For spells that require 2s or longer FCT, it really helps a lot. So yeah the biggest tradeoff here is between Temporal INT and DEX boots, higher MATK vs -0.5s FCT, the question is whether -0.5s FCT reduction is worth the MATK you give up. Its a bit tricky, I am still a High Wizard so I cannot test which scenario is better for a Warlock. I will have to find this out by trying both out a few weeks later.