MediaWiki API imageinfo queries for extended unstructured metadata stored in File pages are very slow because the request involves parsing the info from the page HTML. (I believe the main reason that the /page/media endpoint is so slow is that it involves many such queries.)
@JoeWalsh and I were discussing how it would be handy to have a RESTBase-backed per-image extmetadata endpoint to help speed up extmetadata requests. The endpoint would be something like `/media/image/extmetadata/{title}{/rev}`. This is more of a nice-to-have item than strictly necessary, but would be useful anywhere we have MW API extmetadata queries now, which includes both the feed featured image and the media endpoint in MCS, as well as both native apps' image gallery activities.
To eliminate redundancy, this endpoint would only store and return results for files stored locally to the request domain wiki; the vast majority would of course be on Commons.
@mobrovac / @Pchelolo: What do you think? Is this worth doing, at least as a bridge to when all file metadata properties have been migrated to SDC? Is Cassandra storage availability still a concern?