Why does `git fetch` take a long time to run in GitHub Actions?

Question

I am using GitHub Actions for CI/CD and using actions/checkout@v4 to check out current branch code in the action job. During development, I create a feature branch from develop branch then create a PR to merge to develop branch.

In my build job, I need to compare the changes between feature branch and develop branch, so I need to fetch develop branch as shown in below workflow file:

steps:
      - uses: actions/checkout@v4
      - name: Fetch base branch
        run: |
          git fetch origin ${{ github.base_ref }}
...

The interesting part is that the actions/checkout@v4 took 3 seconds to finish but the git fetch origin develop command took 30 seconds to finish. The two branches are basically similar with only a few changed files. I wonder why it has a lot different when checking out the code. Does GitHub Actions do some optimisation on current branch?

Answer 1

The checkout action uses git fetch --depth=1 by default.