Both pages and posts are identical internally. Pages are posts of type
page, and posts are posts of type
post. It's just a column in the posts table that determines the difference.
Similarly, items in the media library are posts of type
attachment, menu items in a menu are posts of type
nav_menu_item, etc etc
So they're all as fast as eachother. The speed differences come when you start querying for them. E.g. Showing a list of all pages in a column will be expensive if you have 1000 pages. Likewise, the same is true of posts. I mention this because the parent page dropdown in the edit page screen will do this, and it's the one place lots of pages will rear its head as a performance problem.
My advice is to stop repurposing pages, and instead use a custom post type for your content, and a custom taxonomy to organise/structure them. There's no need to have pages that are purely to act as parents for the URL, or to group things. Thats what categories/tags/custom tax are for.