I'm using their official helm chart, it's just idling. Sidekiq is doing nothing, the server is serving no requests - I have no idea how it manages to spend so much time on nothing.
The helm chart "gitlab" is deprecated, the helm chart "gitlab-omnibus" is currently recommended, and the helm chart "cloud native gitlab" is going to be recommended in the near future - a GitLab employee answered me a few hours ago in this thread :)