Listing undefined or orphaned pages doesn't scale

Listing undefined or orphaned pages doesn't scale at all because it is being recomputed on every request with minimal caching. This results in bad user experience or inability to retrieve the information at all (due to network timeouts). We are currently experiencing major problems with spaces that contain only a few hundred pages.

I see in the source code that mysql is being treated differently than other databases, and that implementation of this special case is the source of the inefficiency.