Over time the milestone plugin will gradually slow Jenkins down to a crawl.

This is caused by the org.jenkinsci.plugins.pipeline.milestone.MilestoneStep.xml file.  Each milestone passed and each build finished will require that this file be updated.  In the short term, this is fine, but over time the file will grow quite large.  In my instance, the file reached 55MB.

I have a highly active server which does 1000 jobs/day.  The result was 30MB/sec of disk IO.  This also caused all kinds of blocking issues where jobs where pilling up waiting to update the MilestoneStep.xml file even though the update would be invalidated by the next item in the queue.

The result was it could take a minute to pass a milestone step and 15 minutes to finish a build.