Using this command to rotate an armor stand(when it isn't already facing that direction):

execute as @e[type=armor_stand] at @s run data merge entity @s {Rotation:[90.0f,0.0f]}

or this command:

execute as @e[type=armor_stand] at @s run tp @s ~ ~ ~ 90 ~

will have a gradual visual change by default. If the player wants the change to be instant, then they are out of luck. Since the player has options open to them that allow them to rotate armor stands gradually, but they do not have a way to rotate them instantly, the default behavior should be changed. I believe this only applies to the visual effect, not the actual functionality.

A similar issue occurs when teleporting the armor stand to another location (most notable at a close distance). Using this command on an armor stand with no gravity:

execute as @e[type=armor_stand] at @s run tp @s ~ ~1 ~

will cause a gradual visual update rather than an instant update.