With merge trains the merge requests with a passing feature branch is placed in a queue and tests are run against the combination of that branch and all the branches before it merged in. Since tests will pass 95%+ of the time the feature branches passes this can speed up the amount of merges you can get into master by 10x or more.
In GitLab we made merge trains https://docs.gitlab.com/ee/ci/merge_request_pipelines/pipeli... to solve this problem automatically.
With merge trains the merge requests with a passing feature branch is placed in a queue and tests are run against the combination of that branch and all the branches before it merged in. Since tests will pass 95%+ of the time the feature branches passes this can speed up the amount of merges you can get into master by 10x or more.