loading-states-and-perceived-performance

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# Loading States and Perceived Performance

Users don't mind waiting as much if they understand *what* they are waiting for and *how much* progress is being made. Perceived performance is the design work of making a system feel faster than it actually is.

---

## Choosing the Right Loading State

| Wait Duration | Best Pattern | Use for |
|---|---|---|
| **Short (< 1s)** | **Inline Spinner / Loader** | Button actions, small updates, quick data fetches |
| **Medium (1s – 3s)** | **Skeleton Screen** | Cards, lists, dashboards, profile pages |
| **Long (> 3s)** | **Determinate Progress Bar** | File uploads, complex exports, heavy processing |
| **Full Page** | **Staggered Entry / Animated Sections** | Initial app load, hero sections, immersive transitions |

---

## Simple Cases: Spinners and Loaders

Use spinners for small, contained actions where the layout doesn't change significantly.

- **Button Spinners:** Replace button text or sit alongside it. The button should enter a `disabled` state to prevent double-submissions.
- **Micro-Loaders:** A small 16–24px circle for inline updates (e.g., saving a single field).
- **Animation Tip:** A "spring-loaded" rotation (easing in and out) feels more premium than a constant linear rotation.

```css
@keyframes spin {
  0%   { transform: rotate(0deg); }
  100% { transform: rotate(360deg); }
}
.spinner {
  animation: spin 800ms cubic-bezier(0.4, 0, 0.2, 1) infinite;
}
```

---

## Skeleton Screens (Glimmer/Shimmer)

Skeleton screens provide a visual placeholder that mimics the layout of the final content. This reduces "layout shift" (CLS) and signals to the user exactly where the content will appear.

### The Shimmer Effect
A subtle, moving gradient that travels across the skeleton elements.

```css
.skeleton {
  background: var(--color-grey-100);
  background-image: linear-gradient(
    90deg,
    rgba(255, 255, 255, 0) 0%,
    rgba(255, 255, 255, 0.5) 50%,
    rgba(255, 255, 255, 0) 100%
  );
  background-size: 200% 100%;
  animation: shimmer 1.5s infinite;
}

@keyframes shimmer {
  0%   { background-position: -200% 0; }
  100% { background-position: 200% 0; }
}
```

### Rules for Skeletons
- **Match the shape:** If the final content is a round avatar, use a round skeleton. If it's a 2-line heading, use two bars of varying widths.
- **Stay Recessive:** Skeletons should use your most subtle grey (`--color-grey-100` or `grey-50`). They should not draw focus.
- **Fade into Reality:** When data arrives, fade the actual content in over the skeleton (150–200ms) rather than snapping.

---

## Fully Animated Sections

For major page transitions or initial loads, use a coordinated animation strategy.

### Staggered Entry (Cascading)
Instead of the whole page appearing at once, animate sections in a sequence. This guides the user's eye from the most important content (hero) down to secondary areas.

```css
.section {
  opacity: 0;
  transform: translateY(10px);
  animation: slide-up 400ms ease-out forwards;
}
/* Stagger by index */
.section:nth-child(1) { animation-delay: 100ms; }
.section:nth-child(2) { animation-delay: 200ms; }
.section:nth-child(3) { animation-delay: 300ms; }

@keyframes slide-up {
  to { opacity: 1; transform: translateY(0); }
}
```

### Hero Section "Bloom"
For hero sections, you might use a more complex animation:
1. **Background image** fades in slowly.
2. **Heading** slides in with a slight overshoot (spring).
3. **CTA button** appears last with a crisp fade-in or subtle color transition.

---

## Load in Priority Order — and Prefetch What's Next

Don't wait for everything before showing anything. Load in the order of **value to the user**, so the thing they came for appears first and the rest fills in around it. This is both a perceived-performance win and a code-efficiency one: you fetch and render less up front.

- **First, the highest-value content** — the key figure, the primary record, the above-the-fold answer. Render it the moment it's ready.
- **Then the next tier, then the next** — secondary panels, related lists, and below-the-fold sections stream in behind it (skeletons hold their space so nothing shifts — see the skeleton section above).
- **Fetch only what the current view needs.** Defer data for tabs, drawers, and off-screen sections until they're opened, rather than loading the whole page's worth of data at once.

**Prefetch the predictable next step** — next page, a hovered row's detail, the next wizard step — in the background so it's instant. Don't speculatively load everything; only where there's an obvious next move.

## Advanced: Optimistic UI

The fastest UI is one that doesn't wait for the server at all.
- **The Pattern:** Update the UI immediately assuming the server call will succeed. If it fails, roll back and show an error.
- **Use for:** Liking a post, toggling a switch, renaming a folder, deleting a message.
- **Benefit:** Instant gratification for the user, making the app feel "lightning fast."

---

From dembrandt/dembrandt-skills, MIT.

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