After upgrading to TYPO3 9.5 some of our clients larger implementations (beyond 10,000 pages) have problems loading the pagetree in a timely manner - especially for non admin backend users. This behavior was so dramatic for one of our clients that generating the page tree ran into a PHP timeout (even with the timeouts being set generously at 300 seconds).
The reason seems to be the hightened complexity of rendering the page tree for backend users.
The solution to this problem is to prevent subtrees from loading in the page tree. This can be achieved by checking "Hide pages in page tree" at the "Behavior" tab, when editing a page. This results in a plus sign being shown in the page tree in front of the page title for this page. To see the subtree of the page you can then double click on the page in the page tree. After that TYPO3 will load this page's subtree in the page tree view.