Performance reviewer

Performance Reviewer

Reviews code for performance issues before they show up in production metrics. Measurement-grounded — every finding references a profiler output, an EXPLAIN plan, or a metric from observability-architect. Guesses don't go in the report. Findings table + severity guide + tooling reference in RECIPES.md.

1. When to invoke

• PR touches a hot path (request handler, DB query, loop over large data, queue consumer).
• Endpoint shows up in latency p99 / error-rate dashboards.
• Before a planned load test.
• After a metric regression — dashboard burn, SLO degradation.
• New code with for + await / for + DB call patterns (the N+1 detector).

2. Output format

Same shape as other reviewers — findings table + summary. Sample row layout and severity rubric in RECIPES § 1–2.

Every finding must include a measurement — EXPLAIN ANALYZE, a pprof flame, a histogram bucket, a hyperfine result. "Looks slow" is not a finding.

3. Review approach

1. Measure first. Without a measurement, there's nothing to review. Run EXPLAIN ANALYZE on the suspect query, attach the profiler to the running process, or check the dashboard for the endpoint's p99.
2. Read the diff with measurement in hand. The measurement narrows where to look.
3. Propose fixes that produce another measurement. A fix without "and the new latency is X" is incomplete.

4. What to check — by category

Database — N+1 and unindexed scans

The single most common perf bug. Per sql-architect §5:

• N+1 detection. A list endpoint that fires one query for parents, then one per parent for children. Look for for x in xs: y = repo.get(x.id). Fix: batch with IN (?, ?, ?), a single JOIN, or a DataLoader-style batcher.
• In tests: log every SQL statement; a handler emitting > 3 queries per request is suspect.
• Missing indexes. Per sql-architect §3: index every FK; index columns used in WHERE; composite column order is most-selective-first.
• EXPLAIN ANALYZE shows the truth. Look for Seq Scan on large tables, Sort operations spilling to disk, actual rows >> plan rows (stale stats — run ANALYZE).
• Pagination by OFFSET is O(N). Switch to cursor per sql-architect §4.
• Connection pooling — any handler opening a connection per request is a bug. Pool in the lifespan.

Async — blocking I/O in event loop

The Python async equivalent of N+1.

• time.sleep inside async def — blocks the event loop. Use await asyncio.sleep(d).
• requests.get(...) inside async def — blocking HTTP. Use httpx.AsyncClient.
• Sync DB driver (psycopg2) inside async def — blocking. Use psycopg 3 async.
• Detection: asyncio.get_event_loop().slow_callback_duration = 0.1 in dev logs callbacks over 100 ms.
• asyncio.to_thread(blocking_fn, args) when you genuinely must call a sync API.
• asyncio.TaskGroup, not bare asyncio.gather — handles cancellation and exception aggregation properly per python-architect §4.

Go — goroutine and allocation issues

• Unbounded goroutine spawn. A loop firing go func() per item without backpressure. Use a worker pool or errgroup with SetLimit(n).
• sync.WaitGroup.Go(fn) per go-architect §7 — Go 1.25+, cleaner than Add(1) / defer Done().
• Channels with wrong direction or buffer. Unbuffered + goroutine on each side = deadlock waiting to happen.
• context.Background() in handlers — request context not propagated; cancellation never reaches downstream calls.
• Allocation hot paths. pprof heap profile; if runtime.makeslice / runtime.newobject tops alloc_objects, reuse buffers via sync.Pool.
• Slice growth — appending without make([]T, 0, capacity) reallocates. If you know the size, set it.

Algorithms and data structures

• O(N²) loops over growing collections. Nested for with linear lookups (if x in list) — convert the inner lookup to a set / map.
• sort.Slice in a hot path when items arrive sorted from upstream — trust upstream order.
• Eager materialization of iterators. list(big_generator) defeats the purpose. Stay lazy.
• String concatenation in loops — quadratic in both languages with +. Use "".join(parts) / strings.Builder.
• Regex in hot paths — compile once at module load, never inside the function.

Memory

• Unbounded caches. A dict / map growing without eviction is a memory leak. LRU with max size, or TTL.
• Long-lived references to short-lived data. Closures capture the parent frame; if the closure outlives the function, the frame stays alive.
• Large struct copies. A 2 KB struct passed by value through 10 layers — pointer once size justifies it (~64 bytes is a rough Go cutoff).
• Goroutine leaks — a goroutine waiting on a never-closed channel is a leak. Go 1.26's experimental goroutineleak profile catches them.

Network / external calls

• Missing timeouts — every outbound HTTP / DB / queue / RPC call has a context deadline. Unbounded waits cause cascading failures.
• No retry with backoff — flaky downstream gets hammered in a tight loop. Jittered exponential backoff is mandatory.
• HTTP keep-alive disabled — TLS handshake per request. Reuse the client.
• Single-flight — N concurrent requests for the same key dedupe via singleflight (Go) / a request-scoped cache (Python). Otherwise a cache miss for a hot key floods downstream.

Sampling and observability cost

• High-cardinality labels in metrics — per observability-architect §2: metric explosion is a perf issue for Prometheus too.
• Logging in tight loops — log.Info inside a 1M-iteration loop dominates the runtime. Log once at the end with a summary.
• Tracing every internal call — per-call spans for hot inner loops kill throughput. Trace request boundaries + major sub-ops, not every function.

5. Tooling

Tools that ground the findings — full reference in RECIPES § 3. Always reference the tool output in the Evidence column.

Quick picks: SQL → EXPLAIN (ANALYZE, BUFFERS) + pg_stat_statements. Go → pprof. Python → py-spy. Bench → hyperfine (CLI), go test -bench, pytest-benchmark. Load → wrk / k6. Symptom → Grafana RED+USE.

6. What this skill does NOT do

• Load testing. Generating load is operational work — k6 / wrk runs against staging, not in a code review.
• Capacity planning. "How many instances at 2× traffic" is a different conversation.
• Premature optimization. A finding requires a measurement; gut-feel optimizations are rejected. If the code isn't measurably slow, don't change it.

7. Cross-skill ties

• sql-architect §3–§5, §9 — indexing, query patterns, N+1, EXPLAIN ANALYZE. Most common source of findings.
• observability-architect — RED/USE metrics reveal where to look. Findings often start with "this dashboard shows..."
• grafana-architect — burn rate / SLO dashboards trigger reviews.
• go-architect §7 / python-architect §4 — concurrency primitives.
• rest-api-architect §5 / §14 — cursor pagination, rate limiting at the API layer.
• improve-codebase-architecture — when a finding requires structural change, promote to architecture review.