benchmark

name	benchmark
description	Use after /execute, before /review, when user says "benchmark", "performance", "check speed", or before shipping performance-sensitive changes.
argument-hint	[feature-name] or [URL]

Benchmark: Measure Before You Ship

Philosophy: Performance is a feature. Measure it, track it, and catch regressions before they ship. Bundle size is the leading performance indicator — it's cheap to measure and correlates strongly with load time. Before/after comparison on every PR prevents the slow creep that turns fast apps into slow ones.

Duration targets: BRIEF ~5-10 minutes (bundle size + basic timing), STANDARD ~10-20 minutes (CWV + bundle + comparison), COMPREHENSIVE ~20-40 minutes (full audit + historical trend).

Why This Matters

Performance regressions accumulate silently. Each PR adds "just 50ms" until the app is 3 seconds slower than it was 6 months ago. Without measurement, nobody notices until users complain. Benchmarking on every feature prevents this by catching regressions at the source — before they compound.

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Trigger Conditions

Run this skill when:

• After /execute completes, before /review
• User says "benchmark", "performance", "check speed", "is this slower"
• Before shipping features that affect load time, bundle size, or render performance
• After optimisation work to verify improvement

Do NOT use for:

• Backend-only API performance → use load testing tools
• Database query performance → use query profiling
• Features with no performance surface

Stage Gates — AskUserQuestion

At every PAUSE point in this skill, call the AskUserQuestion tool to present structured options to the user.

For pattern details and examples: ../_shared/references/stage-gates.md

Fallback: Only if AskUserQuestion is not available as a tool (check your tool list), fall back to presenting options as markdown text and waiting for freeform response.

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Mode Selection

Mode	When	Measurements
BRIEF	Quick check, small change	Bundle size comparison only
STANDARD	Typical feature	Bundle size + page load times + Core Web Vitals
COMPREHENSIVE	Performance-sensitive feature	Full audit + historical trend + resource breakdown + budget check

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Collaborative Model

Phase 1: Baseline Capture (if needed)
Phase 2: Current Measurement
Phase 3: Comparison & Regression Detection
  ── PAUSE 1: "Performance report. {N} regressions detected." ──

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Prerequisites

Before starting, verify:

• Build succeeds in production mode (run the project's production build command)
• If measuring page load times, the application dev server is running
• Previous baseline exists at docs/benchmarks/baseline.json or will be captured in Phase 1

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Critical Sequence

Phase 1: Baseline Capture

Step 1.1 — Check for Existing Baseline:

Look for docs/benchmarks/baseline.json or equivalent. If it exists, use it for comparison. If not, capture one from the main branch.

Step 1.2 — Capture Baseline (if needed):

If no baseline exists, measure the main branch state:

Run the project's production build command (check package.json scripts or project CLAUDE.md):

# Build production bundle using project's build command
# Examples: ng build --prod, npm run build, next build, vite build, dotnet publish

Identify the build output directory from the project's build configuration, then measure file sizes of all JS and CSS bundles. Match bundles by chunk type (main, vendor, lazy), not by filename — modern bundlers use content hashing.

Record:

• Bundle sizes (main, vendor, styles, lazy chunks)
• Page load times (if browser available)
• Core Web Vitals (if browser available): LCP, FID/INP, CLS

Save to docs/benchmarks/baseline.json.

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Phase 2: Current Measurement

Step 2.1 — Build & Measure:

Build the current branch in production mode and capture the same metrics as baseline.

Step 2.2 — Core Web Vitals (STANDARD+):

If browser automation is available, measure for each target page using Lighthouse CLI (npx lighthouse {url} --output json --chrome-flags="--headless"), browser DevTools Performance API, or the project's existing performance tools. If no browser automation is available, measure bundle sizes only (fall back to BRIEF mode).

For timing measurements, take the median of 3 runs — network conditions and system load cause variance. Bundle sizes are deterministic and need only one measurement.

Metric	Good	Needs Improvement	Poor
LCP (Largest Contentful Paint)	≤2.5s	2.5-4s	>4s
INP (Interaction to Next Paint)	≤200ms	200-500ms	>500ms
CLS (Cumulative Layout Shift)	≤0.1	0.1-0.25	>0.25

Step 2.3 — Resource Breakdown (COMPREHENSIVE):

Identify the slowest and largest resources:

• Top 10 largest JS bundles
• Top 5 slowest network requests
• Render-blocking resources
• Unused JS percentage

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Phase 3: Comparison & Regression Detection

Step 3.1 — Compare Against Baseline:

Regression thresholds: These are default thresholds. If the project defines performance budgets (in lighthouse config, bundlesize config, or CLAUDE.md), use those instead.

Metric	WARNING	REGRESSION
Bundle size (total)	>10% increase	>25% increase
Bundle size (single chunk)	>15% increase	>30% increase
Page load time	>20% slower	>50% slower or >500ms absolute
LCP	>20% slower	>50% slower or exceeds "Good" threshold
INP	>20% slower	>50% slower or exceeds "Good" threshold
CLS	>0.05 increase	>0.1 increase or exceeds "Good" threshold

Step 3.2 — Build Report:

Save to docs/benchmarks/benchmark-{timestamp}.md:

# Performance Benchmark: {Feature Name}

> **Date:** {date}
> **Branch:** {branch}
> **Mode:** {BRIEF/STANDARD/COMPREHENSIVE}

## Summary

| Verdict | Details |
|---------|---------|
| **Overall** | ✅ PASS / ⚠️ WARNING / ❌ REGRESSION |
| **Regressions** | {count} |
| **Warnings** | {count} |

## Bundle Size Comparison

| Bundle | Baseline | Current | Change | Status |
|--------|----------|---------|--------|--------|
| main.js | {size} | {size} | {+/-}% | ✅/⚠️/❌ |
| vendor.js | {size} | {size} | {+/-}% | ✅/⚠️/❌ |
| styles.css | {size} | {size} | {+/-}% | ✅/⚠️/❌ |
| **Total** | **{size}** | **{size}** | **{+/-}%** | **{status}** |

## Page Load Times (STANDARD+)

| Page | Baseline | Current | Change | Status |
|------|----------|---------|--------|--------|
| {route} | {time}ms | {time}ms | {+/-}% | ✅/⚠️/❌ |

## Core Web Vitals (STANDARD+)

| Metric | Baseline | Current | Change | Rating |
|--------|----------|---------|--------|--------|
| LCP | {time} | {time} | {+/-}% | Good/NI/Poor |
| INP | {time} | {time} | {+/-}% | Good/NI/Poor |
| CLS | {score} | {score} | {+/-} | Good/NI/Poor |

## Resource Breakdown (COMPREHENSIVE)

### Largest Bundles
| Bundle | Size | % of Total |
|--------|------|-----------|
| {name} | {size} | {%} |

### Slowest Resources
| Resource | Time | Type |
|----------|------|------|
| {url} | {time}ms | {js/css/img/api} |

PAUSE 1: Present the summary (verdict, regression count, key metrics) as formatted markdown, then:

AskUserQuestion:
  question: "Performance benchmark complete. {verdict}. How to proceed?"
  header: "Benchmark"
  multiSelect: false
  options:
    - label: "Accept (Recommended)"
      description: "Performance is acceptable. Proceed."
    - label: "Investigate regressions"
      description: "Dig into the specific regressions to find the cause."
    - label: "Optimise"
      description: "Address performance issues before proceeding."
    - label: "Update baseline"
      description: "Accept current measurements as the new baseline."

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Anti-Patterns

Measuring in Development Mode — Development builds include source maps, unminified code, and hot-reload overhead. Always benchmark production builds for meaningful comparisons.

Ignoring Bundle Size — "It's just 50KB more" — every 50KB compounds. Bundle size is the single most predictable performance metric and the easiest to track. Treat it as seriously as test coverage.

Benchmarking Without Baseline — A number without context is meaningless. "Page loads in 1.2s" — is that good or bad? Without a baseline, you can't detect regressions. Always capture before measuring.

Single-Run Measurements — Network conditions and system load vary. For timing measurements, take the median of 3 runs minimum. Bundle sizes are deterministic and need only one measurement.

Moving the Baseline to Hide Regressions — Updating the baseline should be a conscious decision, not an automatic step. The baseline represents the performance contract with users. Update it when you intentionally change performance characteristics, not to make warnings disappear.

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Exit Signals

Signal	Meaning	Next Action
PASS	No regressions	Proceed to /review
WARNING	Minor regressions	Note in review, proceed
REGRESSION	Significant regressions	Investigate and fix before /review

When complete: "Benchmark saved to docs/benchmarks/benchmark-{timestamp}.md. {verdict}."

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Skill Version: 1.0 v1.0: Initial release. Bundle size comparison, Core Web Vitals measurement, regression thresholds, before/after comparison, resource breakdown, baseline management. Inspired by gstack's /benchmark performance engineering patterns.