https://wccftech.com/wow-classic-spell-casts-moved-to-low-priority-loop-to-best-fit-gameplay-in-patch-1-12/

Blizzard has provided additional details on how WoW Classic spell casts will work once the vanilla server option for World of Warcraft launches this summer.

As with other WoW Classic game elements, Blizzard wants to ensure that Classic’s gameplay does not differ from vanilla’s World of Warcraft experience. In current World of Warcraft, spell casts have been moved to a high-priority loop, but for Classic, the team will be processing spell casts will be moved to a low-priority loop that best fits patch 1.12 gameplay.