performance-expert

Performance Expert Skill

Backend, database, and infrastructure performance. This skill owns the server-side and delivery-layer of a stack: request latency, data access, caching, background work, and what ships over the wire.

Render-time work inside React components — re-renders, memoization, virtualized lists, Profiler traces — is a different diagnosis with different tooling. Route it to react-component-performance instead of duplicating it here.

Contract

Inputs:

• A performance symptom with a surface: an endpoint, a query, a job, a build output, or a metric that regressed.

Outputs:

• A ranked list of bottlenecks with the measurement that proves each one, plus a targeted fix per bottleneck.

Creates/Modifies:

• None on its own. It diagnoses and prescribes; edits happen in the caller's workflow.

External Side Effects:

• None. Profiling and load-testing commands are prescribed, not executed unattended.

Delegates To:

• react-component-performance for React render hotspots, re-render churn, memoization, and Profiler-driven component work.
• workspace-performance-audit when the target is a whole monorepo rather than one surface.

When to Use

• Improving API response times
• Optimizing database queries, indexes, and aggregations
• Diagnosing N+1 query patterns
• Implementing caching strategies and invalidation
• Moving heavy work into background jobs or queues
• Profiling a server process or tracing a slow request
• Analyzing shipped bundle size and asset delivery
• Configuring CDN, cache headers, or edge caching

Project Context Discovery

1. Check .agents/memory/ for performance architecture context (e.g., any *performance* or *architecture* files)
2. Identify performance tools (APM, load-testing harness, profiler)
3. Review existing optimizations and caching strategies
4. Check for [project]-performance-expert skill

Core Performance Principles

Backend

API Response Times: Target < 200ms (p95), caching, background jobs, connection pooling

Query Optimization: Indexes, projections (select), pagination, database-side aggregation

Background Processing: Queues for heavy operations, async for non-critical tasks, no blocking work in request handlers

Database

Indexes: On frequently queried fields, compound indexes, monitor usage

Queries: Filter in where, select only needed columns, sort with indexes, avoid sequential scans

Infrastructure

CDN: Edge caching, correct cache headers, static assets off the origin

Serverless: Cold start optimization, memory allocation, provisioned concurrency

Delivery Layer

Bundles: Code splitting by route, dynamic imports for heavy modules, tree shaking, drop unused dependencies

Assets: WebP images, subset fonts, minify CSS/JS, Gzip/Brotli compression

Server rendering: Static generation for static content, incremental revalidation, framework image and font pipelines

Performance Metrics

Backend

• API p50: < 100ms
• API p95: < 200ms
• DB Query p95: < 50ms
• Error Rate: < 0.1%

Delivery

• LCP: < 2.5s
• TTFB: < 800ms
• Initial bundle: < 200KB

Quick Checklist

Backend

• Database queries optimized
• Indexes created and used
• Caching implemented
• Background jobs for heavy operations
• Connection pooling configured

Delivery

• Bundle size < 200KB initial
• Code splitting implemented
• Images optimized and lazy loaded
• Compression and cache headers configured

Before reporting a measured speedup or regression, vet the number with the pstack skill's benchmark checklist reference: name the limiter, confirm every side was tuned, count errors, and repeat the run.

---

For database query optimization code, caching strategy implementation, N+1 query solutions, background-job patterns, infrastructure tuning, performance testing commands, and detailed checklists, see: references/full-guide.md