Right now, the only way for picard to check whether a release is in a particular collection is to fetch the entire contents of the collection, and check if the release is in the list.
In the somewhat theoretical case of me having a collection with all my ~2k releases in it, that means that picard has to spend 20 seconds fetching collections before it can do anything useful.
Ideally, it would be able to easily check whether a release is in a given collection; preferably without doing another query. So the proposal is to add "inc=collections" to the release query, which would include a list of collections containing that release.