title: Why does Jest --runInBand speed up tests?
body: <p>I read that the <a href="https://facebook.github.io/jest/docs/troubleshooting.html#tests-are-extremely-slow-on-docker-and-or-continuous-integration-ci-server" rel="noreferrer">--runInBand</a> flag speeds up Jest test duration by 50% on CI servers. I can't really find an explanation online on what that flag does except that it lets tests run in the same thread and sequentially.</p>
  
  <p>Why does running the test in the same thread and sequentially make it faster? Intuitively, shouldn't that make it slower?</p>