On Jenkins v1.528, after a few hundred builds, web page access and build job completion slows to many times slower than normal, all the way to unusable. Restart fixes the issue. The host machine, a Mac circa 2012, has gigabytes of available memory and CPU usage is normal.
But I notice a persistent spike in disk usage when accessing a Jenkins web page (such as one for a Jenkins job/task/build). It seems that Jenkins has possibly run out of heap space. Yet I can't think of anything that has changed on this Jenkins instance or on the host machine to cause the issue (it didn't used to be a problem).
Also, I've seen some fixes for major slowness recently, but they went into prior versions.