Performance Optimization

Performance Optimization

Overview

Measure before optimizing. Performance work without measurement is guessing — and guessing leads to premature optimization that adds complexity without improving what matters. Profile first, identify the actual bottleneck, fix it, measure again.

When to Use

• Performance requirements exist in the spec (load time budgets, response time SLAs)
• Users or monitoring report slow behavior
• Core Web Vitals scores are below thresholds
• You suspect a change introduced a regression
• Building features that handle large datasets or high traffic

When NOT to use: Don't optimize before you have evidence of a problem.

Core Web Vitals Targets

The Optimization Workflow

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

Where to Start

What is slow?
├── First page load
│   ├── Large bundle? → Measure bundle size, check code splitting
│   ├── Slow server response? → Measure TTFB in Network waterfall
│   └── Render-blocking resources? → Check CSS/JS blocking in waterfall
├── 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
└── Backend / API
    ├── Single endpoint slow? → Profile DaaS queries, check indexes
    ├── All endpoints slow? → Check connection pool, memory, CPU
    └── Intermittent slowness? → Check for lock contention, GC pauses

Fix Common Anti-Patterns

N+1 Queries (DaaS)

// BAD: Separate fetch per related item
for (const task of tasks) {
  task.owner = await fetch(`/api/users/${task.ownerId}`);
}

// GOOD: Use DaaS deep query with fields parameter
const tasks = await fetch('/api/items/tasks?fields=*,owner.*');

Unnecessary Re-renders (React)

// BAD: Creates new object on every 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} />;
}

Large Bundle Size (Next.js)

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

// GOOD: Route-level code splitting (Next.js does this automatically)
// Each page in app/ is automatically code-split

Unbounded Data Fetching

// BAD: Fetching all records
const all = await fetch('/api/items/tasks');

// GOOD: Paginated with DaaS
const page = await fetch('/api/items/tasks?limit=20&offset=0&sort=-date_created');

Performance Budget

JavaScript bundle: < 200KB gzipped (initial load)
CSS: < 50KB gzipped
Images: < 200KB per image (above the fold)
API response time: < 200ms (p95)
Lighthouse Performance score: ≥ 90

Next.js / DaaS Specific Tips

• Use Server Components by default (reduces client JS)
• Use 'use client' only when needed (event handlers, hooks, browser APIs)
• Use next/image for automatic image optimization
• Use next/font for font optimization
• Leverage DaaS aggregate API for counts/sums instead of fetching all records
• Use DaaS fields parameter to fetch only needed fields
• Use DaaS filter parameter server-side instead of client-side filtering

Common Rationalizations

Red Flags

• Optimization without profiling data to justify it
• N+1 query patterns in data fetching
• List endpoints without pagination
• Images without dimensions, lazy loading, or responsive sizes
• Bundle size growing without review
• No performance monitoring in production
• React.memo and useMemo everywhere (overusing is as bad as underusing)

Verification

After any performance-related change:

• Before and after measurements exist (specific numbers)
• The specific bottleneck is identified and addressed
• Core Web Vitals are within "Good" thresholds
• Bundle size hasn't increased significantly
• No N+1 queries in new data fetching code
• Existing tests still pass (optimization didn't break behavior)

See Also

For detailed checklists, see references/performance-checklist.md.