Motion is the part of interface design most likely to be added because it looked good in a portfolio.

What motion is for

• Showing where a new element came from, so it is understood as related rather than as a surprise.
• Maintaining continuity when a layout changes, so the user does not have to re-find their place.
• Confirming that an action registered, in the 100ms before a result arrives.

Practical limits

Keep entrances under 200ms and exits shorter still — an exit animation is pure delay, since the user has already decided. Honour prefers-reduced-motion, which is a genuine accessibility requirement rather than a preference: for some people large motion causes nausea, not annoyance.

And animate transform and opacity rather than layout properties. Animating width or top forces layout on every frame, which on a mid-range phone is the difference between smooth and visibly stuttering.