Good animation answers a question the user was already asking: where did that come from, where did it go, and what just changed?
The test we apply is simple. Remove the animation. If the interface becomes harder to follow, the motion was doing work. If nothing is lost, it was decoration — and decoration costs performance, accessibility, and patience.
Duration matters more than easing curves. Anything over 400ms on a frequent interaction starts to feel like waiting, no matter how elegant the curve.