When you hit a page that has been renamed or deleted, and the page is not in the trash, Confluence does PageNotFound.action, which uses PageManager to find pages that are similar to the original request in other spaces.

PageManager necessarily goes to the database and isn't very efficient. This could lead to an extreme amount of database and memory usage in very large instances.

Since this can cause system instability for large instances, suggest that there be either a commandline flag or an option added to the general configuration that allows you to disable the PageNotFound behavior and instead degrade to simple 404.