Once the move is chosen, it is STILL gameplay, that delay glow effect IS gameplay, its basically a delay just so that it puts up the name of the move at the top of the screen long enough so the player can view it.
even once the player chooses it its STILL much slower then how it would be in the actual story, heres an example
Put it to 8:00, watch boost jump in gameplay
YouTube video
Now watch it in the movie, 6:15, he does it on a whim, much faster then in gaemplay
YouTube video
spell insantly such as Meteor, Blizzaga or barrier the moves are slow like they are in the games.
I would, but they dont show those spells in in the movie or in a canon cutscene
However; casting a summon spell from the negative lifestream happens basically instantly, 8:50
YouTube video
This from CC, Sephiroth uses his power instantly, 0:35
YouTube video
Also keep in mind that Sephiroth/others are not "immobile" while casting a spell, they're free to move, dodge, etc
Now its your turn to show me Sephiroth taking 4+ seconds to use his power OUTSIDE of gameplay.