Improve the performance of DBCollection.findOne()

__find() get "limit" for query. We need only one record, I don't know why that value is 0. If 0 given, mongoD may retrieve all record for result then return the cursor, but when 1 given, immediately return the cursor when found at least 1 record. Isn't it?
see db.collection.find().limit(1).explain() vs. db.collection.find().explain()