WordPress obviously comes from a blogging background but can be used to serve sites with a lot of Pages. However, where it falls short for me is not in regard to performance but in the Admin area’s handling of lots of Pages, child pages etc. It quickly becomes a chore to move through the list of Pages trying to find what you’re looking for, especially without the ability to drill down into page hierarchies etc. What techniques / plugins do people use to deal with this issue?
I use the CMS Tree View plugin, and it works very well. It collapses all your pages under their parent pages, among other features.
Update 26/09/19 This plugin is no longer supported and only tested up to version 4.8.7 of WordPress