In order to spend less time sacrificing, we need the favor to be delivered in less time than it would take to sacrifice a 2nd time after doing the first high-favor cast. Why not make it immediate? I.e. upon sacrificing 200 favor worth of items, fill your favor bar completely and immediately, and retain the excess in a background pool, which would also completely and immediately refill your favor bar, allowing back-to-back casting.
For balance reasons in PvP, this favor pool would need to be tied to a specific location (the altar which was used to sacrifice) or be completely unavailable with enemies in local (can still have favor in your background pool, but can't recharge favor bar). With enemies in local, you should still be able to sacrifice up to your favor bar limit.