Repository mirroring delay - unpredictable sync time?

We setup repository push-based mirroring: gitlab → other_git_repo
It works fine most of the time: git push from laptop to gitlab and then gitlab almost instantly triggers sync to the remote repo.
However from time to time gitlab delayes the sync for a few minutes.

I couldn’t find any patterm why is that (and the git repo is small) - can anyone hints me what might be the problem ?

Does anybody use git push based replication (with Gitlab as a source) and have similar experience ?