I think the main challenge here is the fact that reading lists have items on different projects (so meta data is not centrally available).

Instead of a generator, I would advise adding the meta data directly to the API, preferably via an optional opt-in parameter to keep things backwards compatible.

e.g. http://enwiki.mediawiki.mwdd.localhost:8080/w/api.php?action=query&format=json&list=readinglistentries&rlelists=4&rlesort=updated&rledir=descending&rlelimit=12&formatversion=2&rleprops=description|info|pageimages

@aude to update this task into spike ticket

Change #1287919 had a related patch set uploaded (by Aude; author: Aude):

[mediawiki/extensions/ReadingLists@master] Add proposed ADR: Optimizing API calls for ReadingLists entries

https://gerrit.wikimedia.org/r/1287919

I talked to @Sneha about this issue and she recommended that we exclude pages from outside the project on a list for now and worry about this later.
I've opened T426789: Limit reading lists to pages from the current project to capture the proposal.

Thanks @Jdlrobson-WMF just want to add that this recommendation is based on the assumption that not many people will be saving items cross wiki. And so if this helps with technical debt we can worry about it later if it becomes a real user need in the future.