# Performance

> Performance patterns and anti-patterns for ethereum.org covering TTFB, LCP, INP, CLS, bundle size, build memory, and RSC payload. Use when diagnosing a perf regression or reviewing proposed code changes for perf landmines.

- Skill: `tuyv/performance` (Agent Skill, multi-file: 4 files)
- Install (CLI): `npx skillmds@latest add tuyv/performance`
- Raw SKILL.md: https://api.skillmd.com/api/skills/tuyv/performance/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: tuyv (https://skillmd.com/u/tuyv)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/tuyv/performance

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# Performance Knowledge

Knowledge base. No runtime actions; authoring conventions for maintainers below. Read the relevant `references/*.md` files directly via the Read tool.

## Two entry points

- **Diagnosing** an existing regression → `references/diagnose-table.md` (symptom → bucket).
- **Reviewing** a proposed change before merge → `references/review-checklist.md` (diff pattern → rules to load).

## Cross-cutting anti-patterns

These look right but aren't. Check every review or diagnose pass against this table.

| Looks like…                                                  | Actually…                                                                                                     |
| ------------------------------------------------------------ | ------------------------------------------------------------------------------------------------------------- |
| Adding `CDN-Cache-Control` to fix TTFB                       | Netlify strips them; CF ignores due to `max-age=0`.                                                           |
| Using `unstable_cache` in MDX routes                         | Triggers ISR → runtime 404s. See `edge-caching.md`.                                                           |
| ``await import(`.../${locale}/${ns}.json`)``                 | Webpack enumerates all combinations → OOM. Use `fs.readFile`.                                                 |
| Adding a `useSession` / `cookies()` in root layout           | Every route becomes dynamic. Edge caching dies silently.                                                      |
| `priority: true` via `getImageProps()` on manual `<picture>` | `fetchPriority` attribute is not set; add it manually.                                                        |
| Bumping `revalidate` to hourly to "fix staleness"            | Increases edge miss rate. Keep daily; add client `/api/*` for freshness.                                      |
| Mocking DB/content in perf tests                             | Perf wins must be measured against real prod build (`pnpm build && start`).                                   |
| Removing `common.json` keys without audit                    | Lookups of removed keys silently render the bare key (fallback). Grep the key across `src/` and `app/` first. |

## File map

| Topic                           | File                             |
| ------------------------------- | -------------------------------- |
| Symptom → bucket routing        | `references/diagnose-table.md`   |
| Diff pattern → rules routing    | `references/review-checklist.md` |
| TTFB, edge caching, CDN         | `references/edge-caching.md`     |
| Bundle size, code splitting, RSC payload | `references/bundle.md`  |
| Build OOM, ENOSPC, file tracing | `references/build.md`            |
| INP, main-thread work           | `references/inp.md`              |
| LCP, images                     | `references/images.md`           |

For data-fetching patterns (Trigger.dev tasks, Netlify Blobs, `src/lib/data` caching, internal `/api/*` routes, fetcher retries), use the `data-layer` skill instead.

## Conventions for adding rules

- One topic per file; no content duplication across files.
- Cite a PR (`PR #N`) when a rule comes from a specific shipped change. Add a SHA only when no PR exists.
- Forward-looking ("do this, avoid that"). Keep entries prescriptive — no postmortems or incident narratives.
- New review check? Add a row to `review-checklist.md` AND the matching bucket file. Don't restate the rule inline.