Run light jobs on the smallest runner that fits

Why it works

Runner prices scale with size, but a formatter, a workflow linter or a job that posts a comment uses one core and little memory. On a larger runner, the extra cores sit idle while you pay for them. GitHub-hosted runners also bill each job rounded up to a whole minute, so a 20-second job on an 8-core runner pays a full minute at the 8-core rate. Moving those jobs to the smallest runner that keeps their wall time cuts the bill without slowing anything a developer waits for.

On GitHub-hosted runners the smallest option is ubuntu-slim: 1 vCPU, 5 GB of RAM and 14 GB of disk, x64 only, running as a container instead of a full VM, at a third of the standard 2-core Linux price. Standard runners are free in public repositories, so the savings come in private repositories and on paid runners.

Apply it

Pick the candidates

From the baseline's --json output, which records every job's duration and runner labels, list jobs whose p50 is under a minute or two. Keep the ones that:

• run one single-threaded tool, or only call APIs;
• install nothing, or only a small pinned tool;
• don't use Docker in any form: no container:, no services:, no container actions and no docker commands;
• never come close to 15 minutes.

Leave anything that compiles, runs npm ci or pnpm install, or runs tests in parallel. Those steps use every core they get, so a smaller runner makes them slower.

Move them

jobs:
  pr-title:
    if: github.event_name == 'pull_request'
    runs-on: ubuntu-slim
    timeout-minutes: 5
    steps:
      - name: Check the pull request title
        env:
          TITLE: ${{ github.event.pull_request.title }}
        run: |
          if ! grep -Eq '^(feat|fix|docs|chore|ci|refactor|test|perf)(\(.+\))?!?: ' <<<"$TITLE"; then
            echo "::error::Use a Conventional Commits title, such as 'fix(api): handle an empty body'"
            exit 1
          fi

• timeout-minutes: 5 fails a stuck job well before slim's 15-minute hard limit.
• The title goes through env, not straight into the script, so a crafted title can't inject shell commands.
• The image has Bash, Git, curl, jq, the GitHub CLI, Python, pipx and Node, but no Rust, Go or Java toolchain, and it can't run Docker containers. Check the ubuntu-slim image README before you move a job.

On larger, third-party or self-hosted runners, the same play means stepping down a size: from 8 to 4 vCPU, or from 4 to 2, for jobs that don't use the cores. Self-hosted fleets can size disks the same way: Webflow logged disk use after each job and found many of its CI volumes could be cut by about half.

Measure before you merge

Run the moved job and the original on the same commit, two or three times each, as in the bigger-runner benchmark. Compare median duration and cost per run (billed minutes times the rate). Keep the move if wall time stays about the same.

Keep it safe

• Missing tools cost time. A job that has to install its toolchain first can end up slower than before. Actix's fmt job took 31 and 47 seconds in its first runs on ubuntu-slim, against a historical median of 17, since slim has no Rust toolchain and the job installs one. It may still be cheaper, but check that it isn't on the critical path.
• Hard limits. Slim jobs are terminated and fail at 15 minutes, and 5 GB of RAM can run out for type checkers and bundlers. Jobs that need Docker, privileged operations or file system mounts fail there.
• Concurrency. ubuntu-slim follows the same plan-based concurrency limits as other standard runners, so don't count on it for extra capacity.
• Required checks. Changing runs-on keeps the check name unless the runner label is part of it, such as a matrix job named lint (ubuntu-latest). Update branch protection in the same change if it is.
• Security. Moving a job doesn't change its permissions or secrets. Keep untrusted input like titles and branch names in env, as above.

Verify

• After a week, compare each moved job's p50 and p90 duration with the baseline, and check that none hit its timeout.
• Compare cost per run for those jobs, using billed minutes and the new rate.
• Confirm the same checks reported and passed, with the same check names.