I'm not sure why anyone would say GitLab CI is slow. You run it on any machine you want, so it's fully dependent on that.

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

well gitlab ci is slower on our machine than jenkins. also push based ci might be better for performance when having many workers. while gitlab ci is pull based which can net in negative performance. of course pull based is way less painful when nodes go down/etc..

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.

It's not noticeably slower, and it's really easy to roll out more workers and expand capacity.