perceived-performance

A filling bar reporting how far along a task is, determinate when the end is known and indeterminate when it is not.

A circular progress indicator that fills around a ring, often with the percentage in the middle.

An indeterminate loading indicator that shows work is happening without saying how much of it is left.

The persistent frame of an application, its header, navigation and footer, that stays put while only the content region swaps as you move around.

A container that holds a fixed width to height proportion whatever its size, so media reserves its space before it loads.

Content moving after it has already been painted, usually because something arriving late took space that was never reserved for it.

The pass where script attaches behaviour to server-rendered HTML, turning a page that is already painted and readable into one that answers input.

A list that appends the next page of results automatically as the reader approaches the bottom, so the content never ends and no page controls appear.

Showing the work a system is doing, or stretching it, so the answer feels earned: the search that lists the sites it is checking instead of answering at once.

Deferring the fetch of an image, list section, or module until it is about to be needed, usually when it approaches the viewport.

Rendering only the rows currently in view and a small buffer around them, while spacers preserve the scrollbar of the full list.

A button at the end of a list that fetches and appends the next batch only when pressed, keeping the reader in control of how much arrives.

A persistent notice that the product has lost its connection, saying what still works, what is queued, and when the last sync happened.

Showing the work as it happens, the kitchen made visible, so a wait is understood rather than merely endured and the result is trusted more for having been watched.

Showing the result of an action immediately and reconciling with the server afterward, instead of making the user wait for confirmation.

Showing a cheap stand-in for an image, a blur, a dominant colour, or a low resolution copy, and swapping in the full file once it arrives.

A loading state made of grey shapes standing where the content will be, showing the layout before the data arrives.

Fetching, or fully rendering, a page before the reader asks for it, on a guess about where they go next, so the navigation is already done when the click lands.

Waiting until input has stopped for a set moment before reacting, so a burst of keystrokes or clicks produces one response instead of many.

Keeping the reader's position fixed when content loads or resizes above it, by adjusting scroll offset so the visible text does not jump.

The pause a touch browser once held before dispatching a click, kept open in case a second tap arrived and turned the gesture into a zoom.

Capping how often a continuous stream of input is acted on, so a response fires at a steady rate instead of on every event.

An animation limited to properties the compositor can handle alone, usually transform and opacity, so it keeps running even when the main thread is busy.

How many frames a second the interface actually draws, the budget every animation spends: 60fps leaves about 16ms per frame and a 120Hz display halves it.

Visible stutter in motion caused by frames arriving late or not at all, so an animation that should be continuous reads as juddering.

Reading a geometric value after writing to the DOM in the same frame, forcing the browser to lay out synchronously over and over and stalling the animation that caused it.

How long a spring appears to take, measured to the point the movement stops being visible rather than to its mathematical rest, the figure to quote against a timed animation.

The moving highlight that sweeps across placeholder shapes to signal that content is still on its way.

The typeface actually used when the preferred one is missing or has not loaded, either for the whole run of text or for characters it lacks.

A loading strategy that shows the roman weight as soon as it arrives and lets the remaining styles swap in afterwards, so the swap happens in stages.

The blank period when a browser hides text entirely rather than show it in a fallback face while the web font loads.

Shipping only the characters a page actually needs, cutting the font file down and letting the browser fetch ranges on demand.

The visible swap when a page first paints text in a fallback face and then re-renders it once the web font arrives.

The moment a page paints in the wrong theme before the stored preference is applied, showing a light flash to a reader who chose dark.