Easing is the tone of voice
Duration decides whether motion is noticed. Easing decides how it feels.
Nothing in the world starts at full speed
Linear motion reads as mechanical because no physical object moves that way. An ease-out — fast at the start, settling at the end — reads as responsive, which is why it suits things arriving on screen.
Duration has a narrow window
Under 120ms most transitions are not perceived as motion at all. Over 400ms interface motion starts to feel like waiting. Most of the good work happens between 180 and 300ms.
Match the distance
A small element travelling a short distance needs less time than a full-screen panel. Using one duration everywhere makes small things feel sluggish and large things feel abrupt.
Duration decides whether motion is noticed. Easing decides how it is felt.
Animate the same card entrance three times: linear, ease-out, and a spring. Show a non-designer and ask which one feels like the product is listening.