Ok let me try and spell this out for you cause you have very poor reading comprehension skills.
The game USED to run on a slow tick rate that processed EVERYTHING.
NOW the game runs on a FAST tick rate, but all actions that need to be handled get a priority so not all interations are handled every tick, so they can have much faster tick rates but dont have to spend every tick checking everything.
In Vanilla the slow tickrate that handled spells caused poor interations… so with the faster tick rates they solved those problems, because they didnt want those interations to work that way, they put spells as a high priority(getting checked every tick).
Now to simulate the old behavior they are dropping the priority of spells so they only get checked every few ticks instead of every tick. The servers still run at the faster tick rate they still handle the same amount of spells they just let the spells stack up for a few ticks before actually processing them.
The Classic servers will still run on the faster tick rate and most of the priorities are staying the same, however they are adjusting a little bit to simulate slower tick rates. The game is not going to run at a slower tick rate.