Animation is the only design decision that spends the user’s time directly. A colour choice is free to look at. A four-hundred-millisecond transition costs four hundred milliseconds, every time, forever.
The test
We ask one question of every transition: if this were instant, what would the user lose? There are only a few good answers.
- They would lose track of where an element went
- They would not notice that something changed
- They would think the system had not responded
- They would be startled by a layout shift
If the honest answer is "it would feel cheap," the animation is decoration. Decoration is permitted — occasionally it is the entire point — but it should be budgeted knowingly rather than smuggled in as usability.
Duration is a function of distance
A tooltip travelling eight pixels and a panel travelling nine hundred should not share a duration. We scale roughly with distance and cap the result: nothing in an interface needs longer than half a second unless the user explicitly asked to be entertained.
Fast is not the same as abrupt. Abrupt is a missing easing curve, not a short duration.
Respect the setting
Reduced-motion preferences are not an accessibility checkbox to satisfy at the end. Build the reduced variant alongside the full one, and you will find it clarifies the full one — because it forces you to state what the motion was communicating.