I know the summary is a bit vague but here is the thing. Several times already we have noticed that the build time of certain of our maven jobs increases substantially over the course of several months. We are talking from 30s initially to 3-4 minutes after six months. This happens for jobs configured to run on our solaris build server , but also jobs configured to run on windows slaves.
It is easy to tell a job is getting slow because during maven init it takes a long time to get passed "Scanning for projects" :
As you can see more than 70 seconds during maven init is not normal.
We found out that when we recreate the job (basically just a copy of the job under a different name from the UI, then delete the slow one and rename the copy to the original) performance is back to normal. But after a few months the slowdown becomes noticeable again.
Hope this is sufficient information, we are currently on jenkins 1.625.2 LTS but have observed this with previous versions as well over the last two/three years. Anything else you need to troubleshoot this let me know.