I've failed to meet Ryu-Oh's 18s/turn timer quite a lot, yeah.
And on that note, Vohu Manah's debuff is just 120s, giving you a 20s/turn timer. You can get extra time on Ryu-Oh's timer by having it be the very last thing that you cast, but since you're going to want to use Water res- first... ... well. I guess you can always just forgo the debuff- aspect of Water res- and use it last.
I'd say it's closer to the truth that Vohu Manah's timer is 17s/turn, and Ryu-Oh's BP is 20s/turn. Compare this to the quite standard 180s/8 turns = 22.5s/turn.