perf(build): faster builds via JSON compression skip + zopfli/gen2 tuning

[kennethkalmer]

Context

Faster build feedback for the docs site. The repo already runs the piscina + zopfli compression pool (and CI already sets ASSET_COMPRESSION_ITERATIONS: 1 / COMPRESS_MAX_THREADS: 4), so the easy wins were in. This ports the remaining compression levers from Voltaire.

Changes

1. Skip JSON pre-compression — drop .json from the compress glob. Most are Gatsby's page-data.json (high count, large total) and zopfli-precompressing them was the bulk of onPostBuild time for ~5-10% extra ratio over nginx's live gzip. nginx is already configured with gzip on; gzip_types application/json; gzip_static on; so JSON is live-gzipped at request time — no loss of gzip on JSON responses. assert-compressed.sh updated to match.

2. Default zopfli numiterations 15 → 1 — CI already overrides to 1; this speeds local builds too. The deflate ratio plateaus after the first iteration (<0.5% byte savings vs. 5/15 for 25-45% more time per file). ASSET_COMPRESSION_ITERATIONS still allows bumping for production.

3. Build job on Docker gen2 — only the build job runs the CPU-bound yarn build. gen2 is ~1.4x faster CPU at +20% credits/min. Falls back to xlarge if gen2 is unavailable.

Verification

Local build (NODE_OPTIONS=--max-old-space-size=12288 yarn build):

• Compressing 541 files ... 11.617s (no JSON, default 1 iteration)
• 0 *.json.gz produced; 541 .css/.js/.svg .gz present
• ./bin/assert-compressed.sh passes

CI: confirm build + test-nginx green on gen2.

🤖 Generated with Claude Code

CI impact

First run on this branch vs. last on main: workflow wall-clock 11 min → 7 min (~36%), driven by the build job halving (593s → 299s).

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