Performance Optimization

性能优化

概览

优化前先测量。没有测量的性能工作就是猜测，而猜测会导致过早优化：增加复杂度，却没有改善真正重要的东西。先 profile，识别真实瓶颈，修复它，再次测量。只优化测量证明重要的东西。

何时使用

• Spec 中存在性能要求（加载时间预算、响应时间 SLA）
• 用户或监控报告行为缓慢
• Core Web Vitals 分数低于阈值
• 你怀疑某个变更引入了回归
• 构建需要处理大数据集或高流量的功能

不应使用的情况： 在有问题证据之前不要优化。过早优化增加的复杂度，成本通常超过它带来的性能收益。

Core Web Vitals 目标

优化工作流

1. MEASURE  → Establish baseline with real data
2. IDENTIFY → Find the actual bottleneck (not assumed)
3. FIX      → Address the specific bottleneck
4. VERIFY   → Measure again, confirm improvement
5. GUARD    → Add monitoring or tests to prevent regression

步骤 1: 测量

两种互补方法，两者都要使用：

• Synthetic（Lighthouse、DevTools Performance tab）： 条件可控、可复现。最适合 CI 回归检测和隔离具体问题。
• RUM（web-vitals library、CrUX）： 真实条件下的真实用户数据。用于验证修复是否真的改善了用户体验。

前端：

# Synthetic: Lighthouse in Chrome DevTools (or CI)
# Chrome DevTools → Performance tab → Record
# Chrome DevTools MCP → Performance trace

# RUM: Web Vitals library in code
import { onLCP, onINP, onCLS } from 'web-vitals';

onLCP(console.log);
onINP(console.log);
onCLS(console.log);

后端：

# Response time logging
# Application Performance Monitoring (APM)
# Database query logging with timing

# Simple timing
console.time('db-query');
const result = await db.query(...);
console.timeEnd('db-query');

从哪里开始测量

根据症状决定先测什么：

What is slow?
├── First page load
│   ├── Large bundle? --> Measure bundle size, check code splitting
│   ├── Slow server response? --> Measure TTFB in DevTools Network waterfall
│   │   ├── DNS long? --> Add dns-prefetch / preconnect for known origins
│   │   ├── TCP/TLS long? --> Enable HTTP/2, check edge deployment, keep-alive
│   │   └── Waiting (server) long? --> Profile backend, check queries and caching
│   └── Render-blocking resources? --> Check network waterfall for CSS/JS blocking
├── Interaction feels sluggish
│   ├── UI freezes on click? --> Profile main thread, look for long tasks (>50ms)
│   ├── Form input lag? --> Check re-renders, controlled component overhead
│   └── Animation jank? --> Check layout thrashing, forced reflows
├── Page after navigation
│   ├── Data loading? --> Measure API response times, check for waterfalls
│   └── Client rendering? --> Profile component render time, check for N+1 fetches
└── Backend / API
    ├── Single endpoint slow? --> Profile database queries, check indexes
    ├── All endpoints slow? --> Check connection pool, memory, CPU
    └── Intermittent slowness? --> Check for lock contention, GC pauses, external deps

步骤 2: 识别瓶颈

按类别列出的常见瓶颈：

前端：

后端：

步骤 3: 修复常见反模式

N+1 Queries（后端）

// BAD: N+1 — one query per task for the owner
const tasks = await db.tasks.findMany();
for (const task of tasks) {
  task.owner = await db.users.findUnique({ where: { id: task.ownerId } });
}

// GOOD: Single query with join/include
const tasks = await db.tasks.findMany({
  include: { owner: true },
});

无界数据获取

// BAD: Fetching all records
const allTasks = await db.tasks.findMany();

// GOOD: Paginated with limits
const tasks = await db.tasks.findMany({
  take: 20,
  skip: (page - 1) * 20,
  orderBy: { createdAt: 'desc' },
});

缺少图片优化（前端）

<!-- BAD: No dimensions, no format optimization -->
<img src="/hero.jpg" />

<!-- GOOD: Hero / LCP image — art direction + resolution switching, high priority -->
<!--
  Two techniques combined:
  - Art direction (media): different crop/composition per breakpoint
  - Resolution switching (srcset + sizes): right file size per screen density
-->
<picture>
  <!-- Mobile: portrait crop (8:10) -->
  <source
    media="(max-width: 767px)"
    srcset="/hero-mobile-400.avif 400w, /hero-mobile-800.avif 800w"
    sizes="100vw"
    width="800"
    height="1000"
    type="image/avif"
  />
  <source
    media="(max-width: 767px)"
    srcset="/hero-mobile-400.webp 400w, /hero-mobile-800.webp 800w"
    sizes="100vw"
    width="800"
    height="1000"
    type="image/webp"
  />
  <!-- Desktop: landscape crop (2:1) -->
  <source
    srcset="/hero-800.avif 800w, /hero-1200.avif 1200w, /hero-1600.avif 1600w"
    sizes="(max-width: 1200px) 100vw, 1200px"
    width="1200"
    height="600"
    type="image/avif"
  />
  <source
    srcset="/hero-800.webp 800w, /hero-1200.webp 1200w, /hero-1600.webp 1600w"
    sizes="(max-width: 1200px) 100vw, 1200px"
    width="1200"
    height="600"
    type="image/webp"
  />
  <img
    src="/hero-desktop.jpg"
    width="1200"
    height="600"
    fetchpriority="high"
    alt="Hero image description"
  />
</picture>

<!-- GOOD: Below-the-fold image — lazy loaded + async decoding -->
<img
  src="/content.webp"
  width="800"
  height="400"
  loading="lazy"
  decoding="async"
  alt="Content image description"
/>

不必要的 Re-renders（React）

// BAD: Creates new object on every render, causing children to re-render
function TaskList() {
  return <TaskFilters options={{ sortBy: 'date', order: 'desc' }} />;
}

// GOOD: Stable reference
const DEFAULT_OPTIONS = { sortBy: 'date', order: 'desc' } as const;
function TaskList() {
  return <TaskFilters options={DEFAULT_OPTIONS} />;
}

// Use React.memo for expensive components
const TaskItem = React.memo(function TaskItem({ task }: Props) {
  return <div>{/* expensive render */}</div>;
});

// Use useMemo for expensive computations
function TaskStats({ tasks }: Props) {
  const stats = useMemo(() => calculateStats(tasks), [tasks]);
  return <div>{stats.completed} / {stats.total}</div>;
}

过大的 Bundle Size

// Modern bundlers (Vite, webpack 5+) handle named imports with tree-shaking automatically,
// provided the dependency ships ESM and is marked `sideEffects: false` in package.json.
// Profile before changing import styles — the real gains come from splitting and lazy loading.

// GOOD: Dynamic import for heavy, rarely-used features
const ChartLibrary = lazy(() => import('./ChartLibrary'));

// GOOD: Route-level code splitting wrapped in Suspense
const SettingsPage = lazy(() => import('./pages/Settings'));

function App() {
  return (
    <Suspense fallback={<Spinner />}>
      <SettingsPage />
    </Suspense>
  );
}

缺少缓存（后端）

// Cache frequently-read, rarely-changed data
const CACHE_TTL = 5 * 60 * 1000; // 5 minutes
let cachedConfig: AppConfig | null = null;
let cacheExpiry = 0;

async function getAppConfig(): Promise<AppConfig> {
  if (cachedConfig && Date.now() < cacheExpiry) {
    return cachedConfig;
  }
  cachedConfig = await db.config.findFirst();
  cacheExpiry = Date.now() + CACHE_TTL;
  return cachedConfig;
}

// HTTP caching headers for static assets
app.use('/static', express.static('public', {
  maxAge: '1y',           // Cache for 1 year
  immutable: true,        // Never revalidate (use content hashing in filenames)
}));

// Cache-Control for API responses
res.set('Cache-Control', 'public, max-age=300'); // 5 minutes

性能预算

设置预算并强制执行：

JavaScript bundle: < 200KB gzipped (initial load)
CSS: < 50KB gzipped
Images: < 200KB per image (above the fold)
Fonts: < 100KB total
API response time: < 200ms (p95)
Time to Interactive: < 3.5s on 4G
Lighthouse Performance score: ≥ 90

在 CI 中强制执行：

# Bundle size check
npx bundlesize --config bundlesize.config.json

# Lighthouse CI
npx lhci autorun

另请参阅

详细性能检查清单、优化命令和反模式参考见 references/performance-checklist.md。

常见合理化借口

危险信号

• 没有 profiling 数据支撑的优化
• 数据获取中存在 N+1 查询模式
• 列表 endpoints 没有 pagination
• 图片缺少尺寸、lazy loading 或响应式尺寸
• Bundle size 增长但没有审查
• 生产环境没有性能监控
• 到处使用 React.memo 和 useMemo（过度使用和使用不足一样糟糕）

验证

任何性能相关变更后：

• 有变更前后的测量结果（具体数字）
• 具体瓶颈已识别并处理
• Core Web Vitals 在 "Good" 阈值内
• Bundle size 没有显著增加
• 新的数据获取代码中没有 N+1 查询
• 性能预算在 CI 中通过（如已配置）
• 现有测试仍然通过（优化没有破坏行为）