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

指标	Good	Needs Improvement	Poor
LCP (Largest Contentful Paint)	≤ 2.5s	≤ 4.0s	> 4.0s
INP (Interaction to Next Paint)	≤ 200ms	≤ 500ms	> 500ms
CLS (Cumulative Layout Shift)	≤ 0.1	≤ 0.25	> 0.25

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：

症状	可能原因	排查方式
Slow LCP	大图片、render-blocking resources、慢 server	检查 network waterfall、图片尺寸
High CLS	图片没有尺寸、内容晚加载、字体偏移	检查 layout shift attribution
Poor INP	主线程 JavaScript 过重、大型 DOM 更新	检查 Performance trace 中的 long tasks
Slow initial load	Bundle 过大、网络请求过多	检查 bundle size、code splitting

后端：

症状	可能原因	排查方式
Slow API responses	N+1 queries、缺少索引、未优化查询	检查 database query log
Memory growth	泄漏引用、无界缓存、大 payload	Heap snapshot analysis
CPU spikes	同步重计算、regex backtracking	CPU profiling
High latency	缺少缓存、重复计算、network hops	Trace requests through the stack

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

合理化借口	现实
“以后再优化”	性能债会复利。现在修复明显反模式，把微优化延后。
“在我机器上很快”	你的机器不是用户的机器。要在有代表性的硬件和网络上 profile。
“这个优化显而易见”	如果没有测量，你就不知道。先 profile。
“用户不会注意 100ms”	研究表明 100ms 延迟会影响转化率。用户注意到的比你想的更多。
“框架会处理性能”	框架能避免一些问题，但不能修复 N+1 查询或过大的 bundle。

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）