Count over covered index happens too slow if $in condition exists. Example:

db.forum.topics.count({ "section": ObjectId("56b25ca88d4b465a5dda716e"), "st": { "$in": [ 1, 2] } })

takes ~50ms with 20000 keys examined, while separate queries takes 1 & 0 ms:

db.forum.topics.count({ "section": ObjectId("56b25ca88d4b465a5dda716e"), "st": 1 })

db.forum.topics.count({ "section": ObjectId("56b25ca88d4b465a5dda716e"), "st": 2 })

Not we split such $in queries to mutiple, but:

1. That's not convenient
2. That's impossible for "pagination" requests, when you have $in and skip+limit.