Jenkins startup loads only top items in parallel.

On Startup, the loading of Items is parallelized but only across top level items, not down the hierarchy. Therefore the item loading is very dependent on the Folder structure and that first level. A flat top level structure would load so much quicker than a deep structure.

If you have 4 top level items, the item loading can be parallelized with a maximum of 4 threads. And if one of those folders is small and another huge, the parallelization is pointless.

This could be improved and would considerably decrease the startup time for moderate to large instances.

Reference: https://github.com/jenkinsci/jenkins/blob/jenkins-2.453/core/src/main/java/jenkins/model/Jenkins.java#L3505-L3518