| name | frontend-performance |
| description | Use when modifying layouts, adding assets, or optimizing page speed. Frontend performance patterns for Core Web Vitals (LCP, CLS, INP). Covers font loading, resource preloading, asset scoping, and render-blocking elimination. Not for backend performance — use performance-reviewer instead. |
Frontend Performance
Audience: Developers modifying layouts, adding assets, or optimizing page load
Goal: Prevent performance regressions and improve Core Web Vitals
Core Web Vitals Targets
| Metric | Good | Needs Improvement | Poor |
|---|
| LCP (Largest Contentful Paint) | < 2.5s | 2.5s - 4.0s | > 4.0s |
| CLS (Cumulative Layout Shift) | < 0.1 | 0.1 - 0.25 | > 0.25 |
| INP (Interaction to Next Paint) | < 200ms | 200ms - 500ms | > 500ms |
Font Loading
Font swap causes CLS. Preload critical fonts before stylesheets.
Correct Order in <head>
<link rel="preload" href="/fonts/inter-400.woff2" as="font" type="font/woff2" crossorigin>
<link rel="preload" href="/fonts/inter-700.woff2" as="font" type="font/woff2" crossorigin>
<link rel="stylesheet" href="/application.css">
Rules
- Only preload fonts used above the fold
- Secondary variants (italic, light, extra-bold) stay lazy-loaded
- Always include
crossorigin on font preloads (even same-origin)
- Keep
font-display: swap on all @font-face declarations
@font-face Pattern
@font-face {
font-family: "Inter";
src: url("/fonts/inter-400.woff2") format("woff2");
font-weight: 400;
font-display: swap;
}
Asset Scoping
Before adding any asset (JS, CSS, font) to a shared layout, ask:
"Is this needed on every page or only a subset?"
Decision Table
| Scope | Where to Load |
|---|
| Every page (navigation, auth) | Application layout <head> |
| Admin pages only | Admin layout |
| Single page/feature | Page-specific partial or controller |
| Below the fold | Lazy load or defer |
Render-Blocking Resources
Resources in <head> directly impact LCP. Minimize what loads globally.
<link rel="stylesheet" href="/admin-charts.css">
<link rel="stylesheet" href="/admin-charts.css" media="print" onload="this.media='all'">
<link rel="preload" href="/admin-charts.css" as="style" onload="this.rel='stylesheet'">
Script Loading
<script src="/heavy-lib.js"></script>
<script src="/heavy-lib.js" defer></script>
<script src="/heavy-lib.js" async></script>
| Attribute | When to Use |
|---|
defer | Depends on DOM, order matters (default choice) |
async | Independent script, order doesn't matter (analytics, tracking) |
| None | Must execute before render (rare, avoid) |
Image Optimization
Images are the most common LCP element.
| Technique | Impact |
|---|
loading="lazy" on below-fold images | Reduces initial payload |
fetchpriority="high" on LCP image | Prioritizes hero/banner loading |
Explicit width and height attributes | Prevents CLS from layout reflow |
decoding="async" | Avoids blocking main thread |
<img src="/hero.webp" width="1200" height="600"
fetchpriority="high" decoding="async"
alt="Product screenshot">
<img src="/feature.webp" width="800" height="400"
loading="lazy" decoding="async"
alt="Feature detail">
CLS Prevention Checklist
| Cause | Fix |
|---|
| Images without dimensions | Add width and height attributes |
| Font swap flash | Preload critical fonts, use font-display: swap |
| Dynamically injected content above viewport | Reserve space with min-height or aspect-ratio |
| Ads/embeds without reserved space | Use aspect-ratio container or fixed dimensions |
| Late-loading CSS shifting layout | Inline critical CSS or preload stylesheet |
Audit Patterns
When reviewing code for frontend performance:
For each file in layouts/:
Check <head> for render-blocking resources
Check font preload order (before stylesheets?)
Count globally-loaded assets — flag if > 5 CSS or > 3 JS
For each new asset addition:
Verify scoped to narrowest layout
Verify not duplicated across entrypoints
For each image tag:
Above fold? → needs fetchpriority="high", explicit dimensions
Below fold? → needs loading="lazy"
All images → need width, height, alt, decoding="async"
