Frontend Performance Budget

Frontend Performance Budget

Purpose

Establish enforceable frontend performance budgets and wire them into CI so regressions fail the build instead of shipping. This goes past "LCP is 2.5s good" surface advice: it attributes each Core Web Vital to a concrete cause (render-blocking resource, layout shift source, long task), sets byte budgets per asset class, and measures the real cost of hydration and client-side JavaScript. The output is a budget file plus a Lighthouse CI configuration that other engineers can run locally and in the pipeline.

When to use

• A page feels slow on mid-tier mobile or a real-user-monitoring (RUM) tool reports poor LCP, INP, or CLS at p75.
• A PR adds a large dependency, a hero image, a font, or a third-party script and you need a size/perf impact gate.
• The team ships a SPA or SSR/RSC app and nobody has a JavaScript byte budget.
• Lighthouse scores swing between deploys and there is no CI gate to catch regressions.
• Hydration is suspected of blocking interactivity on a server-rendered page.

When not to use

• The artifact is a backend API or CLI with no browser surface — use backend latency profiling instead.
• The page is an internal tool with a tiny, known audience on fast hardware where the budget would be disproportionate.
• The slowdown is clearly a server response-time problem (high TTFB) rather than a client asset problem — fix the server path first.

Procedure

1. Capture a baseline on throttled mobile. Never measure on an unthrottled desktop; it hides the real cost. Run Lighthouse with mobile emulation and a 4x CPU slowdown, and record LCP, INP (or TBT as a lab proxy), CLS, TTFB, and total transferred bytes by resource type.
2. Attribute each metric to a cause. LCP: which element is the LCP element, and what delays it (TTFB, render-blocking CSS/JS, late-discovered image, font swap)? CLS: which DOM nodes shift and why (image without dimensions, injected banner, web-font reflow)? INP: which interaction is slow and which long task blocks the main thread?
3. Set byte budgets per asset class. Assign hard ceilings: total JS (compressed), CSS, images, fonts, and third-party. A common mobile-first starting point is ~170 KB compressed JS for the initial route, but derive yours from the device and network of the real audience.
4. Audit lazy loading and code splitting. Confirm below-the-fold images use loading="lazy", routes are code-split, and heavy components are dynamically imported. Verify the LCP image is NOT lazy-loaded (that delays it).
5. Measure hydration cost. For SSR/RSC apps, measure how much JavaScript hydrates and how long the main thread is blocked after first paint. Prefer streaming, partial hydration, or server components for static regions.
6. Wire Lighthouse CI with assertions. Add a lighthouserc.js with budget assertions so the pipeline fails when a metric or byte budget regresses.
7. Track at p75 in RUM, not just lab. Lab numbers find regressions; field (RUM) numbers tell you what users actually experience. Reconcile the two.

Concrete checks

• Lighthouse was run with mobile emulation and CPU throttling (not desktop defaults).
• The LCP element is identified and is not lazy-loaded; it is preloaded if late-discovered.
• Every <img> and media element has explicit width/height or an aspect-ratio box (prevents CLS).
• Fonts use font-display: swap (or optional) and are preloaded if critical.
• No render-blocking third-party script sits in the critical path; non-critical scripts are async/defer or loaded post-interaction.
• A per-asset-class byte budget exists and the initial-route JS is within it.
• Routes and heavy components are code-split / dynamically imported.
• A lighthouserc.js (or equivalent) enforces budgets in CI and fails on regression.
• RUM p75 for LCP, INP, and CLS is tracked and reconciled against lab numbers.
• Long tasks (>50 ms) on the main thread during initial interaction are identified.
• Late-loading embeds/ads/iframes have reserved space so they do not cause layout shift.
• The production bundle is minified and tree-shaken (no dev build or source maps shipped to users).

Commands or Templates

# One-off Lighthouse run, mobile + throttling, JSON output
npx lighthouse https://example.com/page \
  --preset=perf \
  --form-factor=mobile \
  --throttling.cpuSlowdownMultiplier=4 \
  --output=json --output-path=./lh-report.json --quiet

# Extract the three Core Web Vitals + total byte weight from the report
node -e '
  const r = require("./lh-report.json");
  const a = r.audits;
  const m = (id) => a[id] && a[id].numericValue;
  console.log("LCP(ms):", m("largest-contentful-paint"));
  console.log("TBT(ms):", m("total-blocking-time"));
  console.log("CLS:", m("cumulative-layout-shift"));
  console.log("TotalBytes:", a["total-byte-weight"].numericValue);
'

# Inspect bundle composition (Vite / Rollup)
npx vite build && npx vite-bundle-visualizer
# or for webpack
npx webpack --profile --json=stats.json && npx webpack-bundle-analyzer stats.json

// lighthouserc.js — Lighthouse CI budget gates
module.exports = {
  ci: {
    collect: {
      url: ['http://localhost:3000/'],
      numberOfRuns: 3,
      settings: { preset: 'perf', formFactor: 'mobile', throttlingMethod: 'simulate' },
    },
    assert: {
      assertions: {
        'largest-contentful-paint': ['error', { maxNumericValue: 2500 }],
        'cumulative-layout-shift': ['error', { maxNumericValue: 0.1 }],
        'total-blocking-time': ['error', { maxNumericValue: 200 }],
        'interactive': ['warn', { maxNumericValue: 3800 }],
        // byte budgets by resource type (bytes)
        'resource-summary:script:size': ['error', { maxNumericValue: 175000 }],
        'resource-summary:image:size': ['warn', { maxNumericValue: 300000 }],
        'resource-summary:font:size': ['warn', { maxNumericValue: 100000 }],
        'resource-summary:third-party:size': ['warn', { maxNumericValue: 150000 }],
      },
    },
    upload: { target: 'temporary-public-storage' },
  },
};

// budget.json — standalone Lighthouse budget (also usable via --budget-path)
[
  {
    "path": "/*",
    "resourceSizes": [
      { "resourceType": "script", "budget": 170 },
      { "resourceType": "stylesheet", "budget": 60 },
      { "resourceType": "image", "budget": 300 },
      { "resourceType": "font", "budget": 100 },
      { "resourceType": "total", "budget": 700 }
    ],
    "resourceCounts": [
      { "resourceType": "third-party", "budget": 10 }
    ]
  }
]

Common issues & anti-patterns

• Measuring on desktop. Unthrottled desktop hides the JS execution cost that dominates on mid-tier mobile; always emulate mobile + CPU throttling.
• Lazy-loading the LCP image. loading="lazy" on the hero image delays the very metric you are trying to improve.
• Treating the Lighthouse score as the goal. The 0–100 score is a weighted composite; optimize the underlying metrics (LCP/INP/CLS), not the number.
• No byte budget, only time budget. Time metrics drift with hardware; a byte budget is a stable, reviewable gate on every PR.
• Shipping the whole bundle to render static content. Hydrating server-rendered static regions wastes main-thread time; use partial hydration / server components.
• Fonts with no font-display and no preload. Causes invisible text then a reflow that spikes CLS.
• Counting only first-load. INP regressions show up during interaction, not initial load — measure interaction latency too.
• Unsized embeds and ads. Late-loading iframes, ad slots, and banners injected above content push everything down and spike CLS; reserve space for them.
• Importing a whole library for one function. Pulling in an entire date or utility library for a single helper bloats the bundle; import the specific function or use a lighter alternative.
• Blocking the critical path on web fonts. Waiting for a custom font before painting text delays LCP; use font-display: swap and a system-font fallback stack.
• Shipping source maps or dev builds to production. Unminified or dev-mode bundles multiply byte weight and parse time; verify the production build is minified and tree-shaken.

Required output

Produce a report containing:

1. Baseline table — LCP, INP/TBT, CLS, TTFB, and bytes-by-type at mobile + throttled, lab and RUM p75 side by side.
2. Attribution — for each failing metric, the specific cause (element, resource, or long task) with file or URL reference.
3. Budget definition — the committed budget.json / lighthouserc.js with per-asset-class ceilings and metric thresholds.
4. Findings — ranked list of regressions or violations, each with the byte/time delta and the concrete fix.
5. CI gate status — whether the pipeline now fails on regression and where the config lives.
6. Next safe action — the single highest-leverage fix (usually the LCP cause or the largest JS chunk).

Safety

• Run Lighthouse and bundle analysis against local or staging builds; do not load-test or hammer production from CI.
• Do not commit secrets or auth tokens into lighthouserc.js collect URLs; use environment variables for authenticated runs.
• Make budget thresholds advisory (warn) first, then promote to error once the team agrees, so you do not block unrelated PRs on day one.
• Do not delete or rewrite existing build configuration without approval; add budget files alongside it.
• Treat third-party script removal as a product decision — flag it, do not silently strip analytics or consent tooling.