GitHub Actions vs GitLab CI vs Jenkins: Which CI/CD Tool in 2025?

GitLab CI runners use a persistent daemon architecture — the runner process stays alive between jobs, eliminating the 20-40 second cold-start overhead GitHub Actions hosted runners incur per job. On self-hosted hardware this compounds: GitLab runners cache Docker layers, node_modules, and Maven repositories on local disk between pipeline runs, while GitHub Actions requires explicit cache configuration with remote storage on every run. For high-frequency pipelines running 500+ times daily on dedicated hardware, this reduces total CI wall-clock time by 30-60%.

Map each Groovy shared library function to a GitHub Actions reusable workflow under `.github/workflows/` and composite action under `.github/actions/`. Migrate pipelines incrementally — keep Jenkins running in parallel and introduce a dual-trigger using the `workflow_dispatch` event and a Jenkins post-build step that calls the GitHub Actions API. Validate output parity on non-production branches before cutting over. The hardest migrations are pipelines that rely on Jenkins' plugin ecosystem for things like Vault secret injection — replace these with OIDC-based authentication and native secret stores before migration, not after.