Question 1

Which curve should I pick?

Accepted Answer

For entering elements a fast-then-slow curve (ease-out) feels right; for exits the reverse. Linear almost always reads as mechanical — the exception being spinning loaders.

Question 2

How long should a transition be?

Accepted Answer

Around 150–200 ms for small state changes and 250–350 ms for larger movements like panels. Past 400 ms the interface starts to feel sluggish.

Easing / Cubic Bezier