Laws of UX · Speed and effort
Doherty Threshold
Productivity soars when a computer and its users interact at a pace (<400ms) that ensures that neither has to wait on the other.
The design-system take
Where the principle shows up in a token, component or pattern decision.
Skeleton states and optimistic UI exist to hold that threshold when the real work takes longer. Perceived speed is a feature you ship, not a metric you measure afterwards.
Adapted from Doherty Threshold on Laws of UX — Jon Yablonski.