Quite possibly a ws/3 request...

One of the annoying limitations of our current webservice is that lookups are limited to a single MBID. That means if you do one lookup and want to fetch more information about some of the entities involved, you end up having to do tons of requests (which then takes forever because of the rate limit).

There would presumably be a limit on the number of MBIDs allowed in a single request, Wikidata for example allows 50 or 500 IDs depending on the request.