On Thursday April 25th, the mw/core.git Jenkins jobs have been slow to report back in Gerrit. Example change: https://gerrit.wikimedia.org/r/#/c/60765/ which took a good 20 minutes for gate-and-submit to report back.
I suspect there were over jobs running at that time such as the parser tests which takes a good 5 minutes to run. So if you have lot of changes submitted (like X of them ), the most recent change would be run (X+1) * 5 minutes after submission.
This is due to the mediawiki-core-phpunit-parser job being shared among all pipelines. The patchsets sent (test pipeline) and the one asked to be merged (gate-and-submit pipeline) ends up racing for an execution slot in Jenkins.