Specifically, CpsFlowExecution holds the loaded scripts, which are potentially large:
And the main script:
We don't need to load these EVERY time we load a build. We could lazy-load them as needed (should happen once in a blue moon I hope).
Reducing the number of times we have to re-persist the build.xml would help even more though - https://issues.jenkins-ci.org/browse/JENKINS-47174