# Frontend Performance — Derived Rules

> Derives from `domains/performance/first-principles.md` P1 (Measure First), P9 (Perceived Performance).

## Measure First (P1)

- Lighthouse, Core Web Vitals (LCP, FID/INP, CLS), RUM (Real User Monitoring).
- A performance claim without a measurement is an opinion.
- Measure the 75th percentile (P75), not the average. The average hides the long tail.

## The Three Core Web Vitals

| Vital | What | Target (P75) |
|-------|------|--------------|
| LCP (Largest Contentful Paint) | When the main content loads | ≤ 2.5s |
| INP (Interaction to Next Paint) | When input is responded to | ≤ 200ms |
| CLS (Cumulative Layout Shift) | Visual stability | ≤ 0.1 |

- LCP > 4s is poor. INP > 500ms is poor. CLS > 0.25 is poor.
- Measure on mobile, not just desktop. Mobile is the long tail.

## Perceived Performance (P9)

- A skeleton screen beats a spinner. A spinner beats nothing.
- Optimistic UI updates: the click responds immediately; the server confirms later.
- Prefetch the next page on hover (if cheap). Prefetch is a bet, not a certainty.

## Bundle Size (P4 Resource Bounds, core C8 Economy)

- Ship less JavaScript. Every KB is parsed, compiled, and executed on the client.
- Code-split routes. Lazy-load below-the-fold. Don't ship the admin bundle to the user bundle.
- A 500KB JS bundle is large. A 2MB JS bundle is a defect.

## Images (P4, P6 Lazy by Default)

- WebP/AVIF, not JPEG/PNG. Modern formats are 30–50% smaller.
- `loading="lazy"` on below-the-fold images. `width`/`height` to prevent CLS.
- `srcset` for responsive images. Ship the right size to the right device.
- Never ship a 4K image to a 360px screen.

## Rendering (P7 Async When Independent)

- Server-render the first paint (SSR/SSG). Hydrate after.
- Avoid hydration waterfalls: a 3-second hydration is a 3-second blank page with a "loaded" script.
- Defer non-critical hydration. Interactive above the fold first; below the fold later.

## What Violates Frontend Performance

| Violation | Principle |
|-----------|-----------|
| 3MB JS bundle | P4, C8 |
| LCP > 4s on mobile | P1 (measured) |
| Layout shift on image load | CLS |
| Synchronous hydration of a 50KB page | P7 |
| No image optimization | P4 |