Performance Review

Measure and evaluate your application's performance against Google's Core Web Vitals thresholds and industry benchmarks. This review catches performance issues that are invisible during development but impact real users — bundle bloat, layout shifts, slow interactions, unoptimized images, and render-blocking resources.

When to use

Use /performance-review when:

• Before launching or after major feature additions
• Page load feels slow but you're not sure why
• Preparing for high-traffic events
• After adding new dependencies or third-party scripts
• SEO rankings depend on performance scores
• Users report slowness or abandonment

Standards Referenced

• Google Core Web Vitals — LCP, INP, CLS (2024 thresholds)
• Google Lighthouse — Performance scoring methodology
• HTTP Archive — Web performance benchmarks (median, p75, p90)
• Web.dev Performance Guidelines — Best practices
• RAIL Model — Response, Animation, Idle, Load budgets

Phase Overview

Phase 1: EDUCATE   → Performance impact on business and what we measure
Phase 2: SCOPE     → Identify key pages, performance budget, baseline
Phase 3: ANALYZE   → Browser-based performance measurement
Phase 4: REPORT    → Findings with metrics, scores, and comparisons
Phase 5: REMEDIATE → Fix guidance + YAML regression tests

Phase 1: Educate

Why this matters: A 1-second delay in page load reduces conversions by 7% (Akamai). Google uses Core Web Vitals as ranking signals since 2021. 53% of mobile visitors leave a page that takes >3 seconds to load (Google). Amazon found every 100ms of latency costs 1% of sales. Performance is a feature — and its absence is a bug.

This review measures real performance in a browser, not just static analysis. We capture actual load times, rendering behavior, and interaction responsiveness.

Phase 2: Scope

Gather context

1. Auto-detect from codebase:

   • Build system (Webpack, Vite, Next.js, etc.)
   • Bundle analysis setup (if any)
   • Image optimization pipeline (sharp, next/image, etc.)
   • Font loading strategy
   • Code splitting configuration
   • Service worker / caching strategy
   • CDN configuration

2. Ask the user (one at a time):

   • Target URL: Where is the app running? (production preferred for realistic measurements)
   • Key pages: Which pages matter most for performance? (recommend: landing page, main feature page, data-heavy page)
   • Performance budget: Any existing targets? (default: Core Web Vitals "Good" thresholds)
   • Known concerns: Any pages that feel slow? (optional)

3. Define measurement plan:

   • Pages to test (3-5 key pages)
   • Conditions: desktop and mobile simulated (Moto G4 / Slow 4G)
   • Metrics: Core Web Vitals + supplementary metrics
   • Baseline: first run establishes baseline for comparison

Phase 3: Analyze

Open a browser session with new_session using record_evidence: true. For each page in scope, run all measurement categories.

Category A: Core Web Vitals (CWV)

Browser validation: Navigate to each page and capture metrics via JavaScript:

// LCP
new PerformanceObserver((list) => {
  const entries = list.getEntries();
  const lcp = entries[entries.length - 1];
  console.log('LCP:', lcp.startTime);
}).observe({ type: 'largest-contentful-paint', buffered: true });

// CLS
let clsValue = 0;
new PerformanceObserver((list) => {
  for (const entry of list.getEntries()) {
    if (!entry.hadRecentInput) clsValue += entry.value;
  }
  console.log('CLS:', clsValue);
}).observe({ type: 'layout-shift', buffered: true });

Category B: Page Load Performance (LOAD)

Browser validation: Use Performance API and performance.getEntries() to gather all metrics.

Category C: Resource Optimization (RES)

Browser validation: Use JavaScript to inspect all loaded resources, their types, sizes, and loading attributes. Use performance.getEntriesByType('resource') for detailed resource metrics.

Category D: Bundle Analysis (BUN)

Browser validation: Use Performance API to measure transfer sizes. Check for source map URLs. Analyze script domain origins.

Category E: Runtime Performance (RUN)

Browser validation: Navigate and interact with the app while measuring performance metrics via JavaScript.

Phase 4: Report

Generate a structured report saved to shiplight/reports/performance-review-{date}.md:

# Performance Review Report
**Date:** {date}
**URL:** {url}
**Pages tested:** {list}
**Conditions:** Desktop + Mobile (simulated Moto G4 / Slow 4G)

## Overall Score: {X}/10 | Confidence: {X}%

## Core Web Vitals Summary
| Metric | Desktop | Mobile | Status |
|--------|---------|--------|--------|
| LCP | 1.8s | 3.2s | ⚠️ Mobile needs work |
| INP | 95ms | 180ms | ✅ Good |
| CLS | 0.05 | 0.15 | ⚠️ Mobile needs work |

## Score Breakdown
| Category | Score | Findings |
|----------|-------|----------|
| Core Web Vitals (CWV) | 6/10 | 1 high, 1 medium |
| Page Load (LOAD) | 7/10 | 1 high |
| Resources (RES) | 5/10 | 2 high, 2 medium |
| Bundle (BUN) | 6/10 | 1 high, 1 medium |
| Runtime (RUN) | 8/10 | 1 medium |

## Resource Waterfall
(Top 10 slowest resources with load times)

## Bundle Breakdown
| Category | Size (gzipped) | Budget | Status |
|----------|---------------|--------|--------|
| First-party JS | 180KB | 250KB | ✅ |
| Third-party JS | 220KB | 150KB | ❌ Over budget |
| CSS | 45KB | 100KB | ✅ |
| Images | 1.2MB | 1.5MB | ✅ |
| Fonts | 85KB | 100KB | ✅ |

## Findings
(structured findings with metrics and evidence)

Confidence Scoring

• 90-100%: Measured in browser with specific values (e.g., LCP: 3.2s)
• 70-89%: Derived from resource analysis (e.g., unoptimized images detected)
• 50-69%: Code-level pattern (e.g., no lazy loading attributes found)
• Below 50%: Don't report

Phase 5: Remediate

1. Fix guidance (example)

#### RES-01: Images not using modern formats
**Impact:** ~40% larger images than necessary, adds ~500KB to page weight
**Current:** 8 PNG images totaling 1.2MB
**Fix:** Convert to WebP with fallback:
- Use `<picture>` with WebP source and PNG fallback
- Or use Next.js `<Image>` / `sharp` for automatic format negotiation
- Expected savings: ~480KB (40% reduction)
**Priority files:**
- /images/hero.png (320KB → ~190KB as WebP)
- /images/features.png (280KB → ~165KB as WebP)

2. YAML regression test

- name: cwv-01-lcp-under-threshold
  description: Verify Largest Contentful Paint is under 2.5 seconds
  severity: high
  standard: Core-Web-Vitals-LCP
  steps:
    - description: Set up an LCP observer before navigation
      js: |
        // Set up LCP observer before navigation
        await page.evaluateOnNewDocument(() => {
          window.__lcp = 0;
          new PerformanceObserver((list) => {
            const entries = list.getEntries();
            window.__lcp = entries[entries.length - 1].startTime;
          }).observe({ type: 'largest-contentful-paint', buffered: true });
        });
    - URL: /
    - WAIT_UNTIL: Page has fully loaded including all images and content
      timeout_seconds: 30
    - description: Assert Largest Contentful Paint is under 2.5s
      js: |
        const lcp = await page.evaluate(() => window.__lcp);
        if (lcp > 2500) {
          throw new Error(`LCP is ${lcp}ms, exceeds 2500ms threshold`);
        }
        console.log(`LCP: ${lcp}ms (threshold: 2500ms)`);
    - VERIFY: Page loaded with Largest Contentful Paint under 2.5 seconds

Save all YAML tests to shiplight/tests/performance-review.test.yaml.

Depth Levels

• --quick: Core Web Vitals only on the main page. ~2 minutes.
• default: All categories on key pages, desktop + mobile. ~8-12 minutes.
• --thorough: All categories + extended pages + multiple runs for statistical confidence + runtime profiling. ~20-30 minutes.

Tips

• Measure on production (or production-like build) — dev mode performance is misleading
• Run multiple times — performance measurements vary; look for patterns, not single data points
• Mobile simulation reveals issues that desktop hides — always test both
• Use performance.getEntries() — it's the richest source of performance data in the browser
• Focus on Core Web Vitals first — they're the metrics Google uses for ranking
• Close session with close_session and use generate_html_report for evidence