Slow meteor query

slow_query.js

if (Meteor.isClient) {

    Meteor.startup(function () {

        var messageThreads = new Mongo.Collection(null);

        for (var i = 0; i < 1500; ++i) {
            messageThreads.insert({
                _id: new Mongo.ObjectID().toHexString(),
                "posts": [
                    {
                        "_id": "91cb147f7b755809342931e1",
                        "header": {
                            "sender": {
                                "_id": "a18a219f0e28ddf75f9cff29",
                                "links": {
                                    "userId": "9f72v97wrb9ewfvg4"
                                },
                                "givenName": "test",
                                "familyName": "tester",
                                "emailAddresses": [
                                    "test@example.com"
                                ],
                                "type": "contact",
                                "_meta": {
                                    "ownerId": "eipfh24pirnwbipfngw",
                                },
                                "pictureUrl": "https:\/\/lh3.googleusercontent.com\/-VMJsZ9uULto\/AAAAAAAAAAI\/AAAAAAAAABM\/29fb9f9439\/photo.jpg"
                            },
                            "recipients": [],
                            "postedAt": "2016-01-01T00:00:00.000Z",
                            "wasRead": true,
                            "isDraft": false
                        },
                        "content": "<span>aaa<\/span>",
                        "needsDigest": false,
                        "links": {
                            "threadId": "c420fc7e241fba9cf12b4b27",
                        },
                        "_meta": {
                            "ownerId": "eipfh24pirnwbipfngw",
                            "deletedAt": 1447206557932
                        }
                    }
                ],
                "_meta": {
                    "ownerId": "6ErzSiTeqwo9zQi8W",
                    "createdAt": "2015-11-11T01:36:33.225Z"
                },
                "lastLivePostedAt": "2016-01-01T00:00:00.000Z"
            });
        }

        var thread = messageThreads.findOne();

        // Slow query (no projection, uses minimongo sort).
        console.time('fetch full');
        var threads = messageThreads.find({ 'posts._meta.deletedAt': { $exists: false } }, { sort: { lastLivePostedAt: 1 } }).fetch();
        _.sortBy(threads, function (t) { return t.lastLivePostedAt });
        console.timeEnd('fetch full');


        // Fast query (projection, no minimongo sort).
        console.time('fetch optimised');
        var threads = messageThreads.find({ 'posts._meta.deletedAt': { $exists: false } }, { fields: { 'posts._id': 1, 'lastLivePostedAt': 1 } }).fetch();
        _.sortBy(threads, function (t) { return t.lastLivePostedAt });
        console.timeEnd('fetch optimised');

        console.time('fetch id');
        messageThreads.findOne(thread._id);
        console.timeEnd('fetch id');

    });

}