Review Performance

Performance analysis for common bottlenecks and inefficiencies.

Usage

/review-perf                  # Review context-related code
/review-perf --staged         # Review staged changes
/review-perf --all            # Full codebase audit (parallel agents)

Scope

Flag	Scope	Method
(none)	Context-related code	Files from the current conversation context: any files the user has discussed, opened, or that you have read/edited in this session. If no conversation context exists, ask the user to specify files or use --staged/--all.
--staged	Staged changes	git diff --cached --name-only
--all	Full codebase	Glob source files, parallel agents

Gotchas

• Default scope (no flag) uses conversation context, which may be stale from an earlier part of the session. The review silently targets the wrong files if context has shifted.
• Blindly adding useMemo/useCallback to fix re-render warnings can worsen performance on simple components — memoization has overhead and only helps when the memoized value is expensive or the child does reference equality checks.

Workflow

1. Determine scope based on flags (see Scope table above)
2. Review each file against all 5 categories in the Performance Checklist below: Algorithmic Complexity, Database/Query Patterns, Memory Management, UI/Render Performance, Network/IO
3. Parallelize if scope has >5 files: spawn one sub-agent per category, each scanning all files for that category. Merge results and deduplicate.
4. Classify severity for each finding:
   • Critical: User-facing slowdown, data loss risk, or resource exhaustion (e.g., memory leak, N+1 on hot path)
   • High: Measurable inefficiency on a common code path but not immediately user-visible (e.g., O(n²) on lists typically < 100 items but growing)
   • Medium: Suboptimal pattern that could become a problem at scale (e.g., missing pagination, sequential requests that could be parallel)
   • Suggestion: Optimization opportunity with marginal current impact
5. Report findings grouped by severity using the Output Format below

Performance Checklist

Algorithmic Complexity

• Nested loops over the same collection (O(n²) or worse) — replace with a lookup map
• Repeated expensive calculations inside a loop — hoist outside
• Array scans that could exit early — use .find() or equivalent

Database/Query Patterns

• Queries inside loops (N+1) — use eager loading / batch fetch
• Loading all rows without a LIMIT — add pagination
• SELECT * when only a few columns are needed — select explicitly
• Missing indexes on columns used in WHERE, ORDER BY, JOIN, or foreign keys

Memory Management

• Connections or file handles opened without a finally/close — always close in finally
• Caches with no size or TTL bound — use LRU or TTL-bounded cache
• Event listeners added without a corresponding removal — return cleanup in useEffect

UI/Render Performance

• Inline object/function literals passed as props causing reference churn — memoize with useMemo/useCallback
• Long lists rendered without virtualization — use a virtualized list component
• Heavy synchronous computation on the main thread — offload to a web worker or chunk the work

Network/IO

• Sequential await calls for independent requests — use Promise.all
• The same request fired multiple times without deduplication — use SWR/React Query or a request cache

For annotated BAD/GOOD code examples for each category, see references/perf-checklist.md.

Output Format

## Performance Review: {scope}

### Critical (user-facing slowdown)
- {file}:{line} - {issue type}: {description}
  **Impact:** {why it matters}
  **Fix:** {solution with code example}

### High Priority
- {file}:{line} - {issue}
  **Fix:** {solution}

### Medium Priority
- {file} - {issue}

### Suggestions
- {optimization opportunity}

Examples

Staged changes introduce N+1 query:

/review-perf --staged

Reviews staged files and catches a new user list endpoint that queries posts per user in a loop. Reports it as Critical with the impact ("100 users = 101 queries") and provides a fix using eager loading with include.

Full audit finds memory leak in dashboard:

/review-perf --all

Parallel agents scan the full codebase by category. Finds an event listener in the dashboard component that is never cleaned up on unmount, plus an unbounded in-memory cache growing with every API call.

Troubleshooting

False positive on a rarely-executed code path

Solution: If the flagged code runs only during initialization or in admin-only flows, note the expected data size in a code comment. Re-run the review and the context will help distinguish hot paths from cold ones.

Cannot determine algorithmic complexity without runtime data

Solution: Add a brief comment with the expected input size (e.g., // n is typically < 50) so static analysis can assess impact. For uncertain cases, use /perf-test to measure actual performance with realistic data.

Notes

• Focus on measurable impact, not micro-optimizations
• Consider data size - O(n²) on 10 items is fine, on 10,000 is not
• For --all, use parallel agents per category
• Database issues often have the highest impact
• UI issues matter most for user-facing code