---
name: ln-31-performance-optimizer
description: "Optimizes a measured latency, throughput, memory, CPU, or I/O problem through profiling and keep-or-discard experiments. Use for a known bottleneck; not unbiased A/B comparison."
---

# Performance Optimizer

**Goal:** Optimize only measured problems. Preserve correctness, isolate experiments, and retain a change only when comparable evidence shows that it improves the agreed metric without unacceptable regressions.

**Execution contract:** Treat the ordered checkbox workflow below as this skill's Definition of Done. Work through every item in order, and mark it complete only when its action and required evidence are complete. `N/A`, skipped, unavailable, or delegated items remain incomplete.
Before returning, apply this skill's verdict, decision, and approval rules to every incomplete item and prepend **Checklist: X/Y complete**<br>**Incomplete: None | section/item — reason; outcome impact; exact next action**; list every incomplete item.

## Tool Routing

| Need | Preferred tool | Use it when | Fallback |
|---|---|---|---|
| Repository state and safe edit boundary | Git status, diff, branch or worktree inspection, and repository instructions | Always before profiling or editing | Stop if user changes cannot be isolated safely |
| Baseline and final metric | Existing benchmark, load test, reproducible command, or production-like replay | The metric and workload reflect the reported problem | Create the smallest local benchmark that reproduces the behavior without inventing production scale |
| Bottleneck evidence | Existing profiler, tracing, query diagnostics, allocation tools, or OS-level metrics | Locating CPU, memory, I/O, lock, query, network, or scheduler cost | Targeted instrumentation with cleanup plan |
| Code path and blast radius | Language server or host-native code intelligence | Following hot symbols, callers, implementations, and affected contracts | Narrow search plus direct inspection of definitions and consumers |
| Correctness and regressions | Repository-defined tests, build, lint, type, and smoke commands | Before and after every retained experiment | Add a focused safety test when current coverage cannot detect the likely regression |
| Runtime and dependency semantics | Official documentation, release notes, and specifications matching installed versions | A hypothesis depends on optimizer, runtime, database, framework, or library behavior | Primary-source web research; otherwise mark the hypothesis `UNVERIFIED` |
| Independent challenge | One native subagent or advisor when policy and scope allow | Competing hypotheses, unfamiliar runtime behavior, or high-risk change needs independent scrutiny | Separate adversarial hypothesis review |

Do not optimize by aesthetic preference or benchmark a different workload from the reported problem. Never discard user changes, use destructive Git reset, or run uncontrolled load against production.

## Evidence Rules

- Define the primary metric and acceptance threshold before editing; do not move the goal after seeing results.
- Compare the same workload, environment, build mode, data, warmup, and measurement method before and after.
- Use repeated runs and report distribution or variance; a single faster run is not proof.
- Separate cold-start, warm steady-state, and saturated-load behavior when the reported problem can occur in more than one regime.
- Profile contribution and end-to-end impact separately: a hot function can improve while the user-visible metric does not.
- Treat profiler estimates, synthetic workloads, and production observations as different evidence classes and label them.
- Correctness, resource safety, and operational stability are hard constraints, not secondary metrics.

## Checklist

### 1. Define the Problem and Protect the Workspace

- [ ] Resolve the user-visible problem, affected workflow, target metric, workload, environment, constraints, and acceptance threshold.
- [ ] Run the wrong-tool gate: confirm the symptom is a performance problem rather than correctness, configuration, capacity, dependency, or observability failure.
- [ ] Read repository instructions and inspect Git state, branches, uncommitted changes, ignored artifacts, and available isolation mechanisms.
- [ ] Preserve user work and isolate experiments in a safe branch or worktree when changes, benchmarks, or generated artifacts could interfere.
- [ ] Start a run-owned resource ledger with every created absolute path, worktree, process ID, cache, profile, and temporary artifact; never register pre-existing resources as cleanup targets.
- [ ] Identify correctness, security, memory, cost, compatibility, and operational constraints that no optimization may violate.
- [ ] Locate existing benchmarks, profiles, performance tests, production traces, service-level objectives, and known environmental variability.

### 2. Establish a Reproducible Baseline

- [ ] Use the same metric type as the observed problem: latency distribution, throughput, CPU, memory, allocation, I/O, query count, lock wait, or another direct measure.
- [ ] Make the workload representative and deterministic enough to compare, including data size, concurrency, cache state, warmup, and build mode.
- [ ] Cover the operating points that could reverse the conclusion--at minimum the reported case plus relevant data-size or concurrency boundaries--without inventing synthetic scale.
- [ ] Run enough repetitions to expose noise and record raw results, median or appropriate percentile, spread, failures, and environment details.
- [ ] When drift or noise is material, interleave or randomize baseline and candidate runs and prefer paired comparisons over one block of "before" followed by one block of "after."
- [ ] Verify that the benchmark detects an intentionally slower or obviously changed path when practical; a benchmark insensitive to behavior cannot validate optimization.
- [ ] Run relevant correctness tests before editing so pre-existing failures are not attributed to experiments.
- [ ] Stop and report `BLOCKED` if the problem cannot be reproduced and no trustworthy production evidence can define a safe proxy.

### 3. Profile and Form Hypotheses

- [ ] Profile the end-to-end path before focusing on a function, query, allocation, lock, or network call.
- [ ] Build a ranked cost map with measured contribution, call frequency, inclusive and exclusive cost where available, and affected workload.
- [ ] Trace the top costs to implementation, callers, data shape, concurrency model, configuration, and external dependencies.
- [ ] Distinguish root bottlenecks from downstream symptoms, measurement overhead, debug builds, cold starts, and one-time initialization.
- [ ] If profiling crosses services or processes whose code is in scope, align traces/correlation IDs and follow the measured downstream path; do not label an accessible internal service "external" and stop at its latency.
- [ ] Estimate profiler or instrumentation perturbation and confirm the final end-to-end result without invasive instrumentation.
- [ ] Research official runtime, framework, database, and dependency behavior only when it can confirm or reject a concrete hypothesis.
- [ ] Check existing platform and dependency capabilities before proposing custom caches, pools, schedulers, serializers, or data structures.
- [ ] Write a small ordered hypothesis set; for each state expected metric change, mechanism, affected files, risk, dependencies, and verification.
- [ ] Reject hypotheses that lack a measurable mechanism, require speculative scale, or cannot be rolled back independently.
- [ ] Use an independent challenge when competing explanations remain plausible; verify advisor claims before changing code.

### 4. Execute Atomic Keep-or-Discard Experiments

- [ ] Add or identify a safety test for behavior most likely to regress before applying each risky hypothesis.
- [ ] For caching, batching, parallelism, pooling, or retry changes, explicitly protect invalidation, ordering, idempotency, cancellation, backpressure, timeout, and bounded-resource semantics that the faster path could violate.
- [ ] Apply the smallest coherent change that tests one mechanism; group changes only when their effects are intentionally inseparable.
- [ ] Keep instrumentation bounded, low-overhead, and easy to remove; never leave secrets or sensitive payloads in diagnostic output.
- [ ] Run focused correctness checks immediately after the edit and discard the experiment if they fail.
- [ ] Repeat the exact baseline benchmark under comparable conditions and preserve raw results.
- [ ] Inspect the diff for accidental cleanup, unrelated refactoring, generated churn, debug flags, changed benchmark inputs, and hidden configuration changes.
- [ ] Mark the experiment `KEEP` only if the agreed metric improves beyond noise and all constraints pass.
- [ ] Mark it `DISCARD` and revert only that experiment when the target is missed, results regress, or safety becomes uncertain.
- [ ] After a kept change, establish the new compound baseline before testing the next hypothesis.

### 5. Stop, Verify, and Report

- [ ] Continue only when new measurement supports another hypothesis; stop at the target, diminishing returns, exhausted safe options, or a missing prerequisite.
- [ ] Run the full relevant build, lint, type, test, smoke, benchmark, and operational checks on the final retained state.
- [ ] Remove only run-owned ledger entries: verify absolute paths remain inside approved temporary roots, stop exact recorded process IDs, preserve dirty or pre-existing worktrees, and retain evidence artifacts intentionally reported.
- [ ] Confirm that the benchmark definition and acceptance threshold did not drift during the run.
- [ ] Report every kept and discarded hypothesis, not only the successful path, so future work does not repeat disproven experiments.
- [ ] Use `IMPROVED` only when at least one retained change improves the agreed metric beyond noise with every constraint passing; use `NO_CHANGE` when all experiments are discarded and the baseline is restored; use `BLOCKED` when a safety prerequisite, reproducible baseline, or safe restoration path is unavailable.
- [ ] Return the verdict with baseline, final distribution, delta, correctness evidence, limitations, and residual bottlenecks.

## Output Contract

```markdown
# Performance Optimization

**Verdict:** IMPROVED | NO_CHANGE | BLOCKED

## Target and method
- User-visible problem and workload
- Primary metric and acceptance threshold
- Environment, repetitions, and variance method
- Correctness and operational constraints

## Baseline and profile
| Metric | Baseline | Variance | Evidence |
|---|---:|---:|---|
| ... | ... | ... | ... |

## Experiments
| Hypothesis | Change | Result | Decision | Verification |
|---|---|---|---|---|
| ... | ... | ... | KEEP / DISCARD | ... |

## Final result
Comparable before/after metrics, full verification, cleanup, limitations, and residual bottlenecks.

## Evidence artifacts
Run-owned paths and hashes for raw samples, commands, environment capture, final diff, and cleanup proof.
```