Yeah, being unable to host Gitlab under 512MB is just too bloaty for me. I know Gogs doesn't have this issue for the same features I use. I still use Gitlab though because it's the mainstream.
and we also use it for their CI integration. It's just simpler than setting up a Jenkins instance for simple jobs (well gitlab ci is slower but not so much that it is painful)
The script that authenticates and picks up the job is a small Golang app. That's all. GitLab executes CI jobs immediately.
See the architecture and related links here: https://about.gitlab.com/gitlab-ci/#architecture
actually the biggest problems of slowness also comes from docker and caching. I think our docker setup for jenkins/gitlab-ci is not equal. for some jobs we will probably migrate away from docker, as soon as we are able to have a good story for running parallel tests.