How to Speed Up Jest in CI

Jest is fast locally but stumbles in CI when worker counts and caching are wrong. A few flags recover most of the gap.

Jest parallelizes across worker processes and caches transformed modules. In CI the defaults often misjudge the host, so setting workers, sharding, and persisting the cache matter.

1. Set workers explicitly

CI runners report misleading CPU counts. Pin --maxWorkers to the real core count to avoid oversubscription thrash.

- run: npx jest --maxWorkers=4 --ci

2. Shard across machines

Jest supports native sharding to split the suite across runners.

strategy:
  matrix:
    shard: [1/3, 2/3, 3/3]
steps:
  - run: npx jest --shard=${{ matrix.shard }} --ci

3. Persist the Jest cache

Cache the transform cache directory so transpilation is reused across runs.

- uses: actions/cache@v4
  with:
    path: /tmp/jest-cache
    key: jest-${{ hashFiles('package-lock.json') }}
- run: npx jest --cacheDirectory=/tmp/jest-cache --ci

4. Run only affected tests on PRs

Use jest --changedSince=origin/main on PR runs so the suite scales with the diff, not the repo. Keep the full run on main.

Key takeaways

• Pin --maxWorkers to the real core count to avoid oversubscription.
• Use --shard to spread tests across runners.
• Persist the Jest transform cache and run --changedSince on PRs.

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