是什么让我的投影变慢？

Question

不知怎的，投影会让我的查询变慢。

这是我的疑问：

{
    "$and": [{
        "$or": [{
            "tiles": "18-137068-57860"
        }, {
            "tiles": "18-137068-57861"
        }, {
            "tiles": "18-137068-57862"
        }, {
            "tiles": "18-137068-57863"
        }, {
            "tiles": "18-137068-57864"
        }, {
            "tiles": "18-137068-57865"
        }, {
            "tiles": "18-137068-57866"
        }, {
            "tiles": "18-137069-57860"
        }, {
            "tiles": "18-137069-57861"
        }, {
            "tiles": "18-137069-57862"
        }, {
            "tiles": "18-137069-57863"
        }, {
            "tiles": "18-137069-57864"
        }, {
            "tiles": "18-137069-57865"
        }, {
            "tiles": "18-137069-57866"
        }, {
            "tiles": "18-137070-57860"
        }, {
            "tiles": "18-137070-57861"
        }, {
            "tiles": "18-137070-57862"
        }, {
            "tiles": "18-137070-57863"
        }, {
            "tiles": "18-137070-57864"
        }, {
            "tiles": "18-137070-57865"
        }, {
            "tiles": "18-137070-57866"
        }, {
            "tiles": "18-137071-57860"
        }, {
            "tiles": "18-137071-57861"
        }, {
            "tiles": "18-137071-57862"
        }, {
            "tiles": "18-137071-57863"
        }, {
            "tiles": "18-137071-57864"
        }, {
            "tiles": "18-137071-57865"
        }, {
            "tiles": "18-137071-57866"
        }, {
            "tiles": "18-137072-57860"
        }, {
            "tiles": "18-137072-57861"
        }, {
            "tiles": "18-137072-57862"
        }, {
            "tiles": "18-137072-57863"
        }, {
            "tiles": "18-137072-57864"
        }, {
            "tiles": "18-137072-57865"
        }, {
            "tiles": "18-137072-57866"
        }, {
            "tiles": "18-137073-57860"
        }, {
            "tiles": "18-137073-57861"
        }, {
            "tiles": "18-137073-57862"
        }, {
            "tiles": "18-137073-57863"
        }, {
            "tiles": "18-137073-57864"
        }, {
            "tiles": "18-137073-57865"
        }, {
            "tiles": "18-137073-57866"
        }, {
            "tiles": "18-137074-57860"
        }, {
            "tiles": "18-137074-57861"
        }, {
            "tiles": "18-137074-57862"
        }, {
            "tiles": "18-137074-57863"
        }, {
            "tiles": "18-137074-57864"
        }, {
            "tiles": "18-137074-57865"
        }, {
            "tiles": "18-137074-57866"
        }]
    }, {
        "lastupdate": {
            "$gte": 0
        }
    }]
}

这是我的预测：

{
    "_id": 1,
    "_owner": 1,
    "destination": 1,
    "endtime": 1,
    "position": 1,
    "speed": 1,
    "starttime": 1,
    "angle": 1,
    "mapspeed": 1,
    "vehicleClass": 1,
    "target": 1,
    "lastupdate": 1,
    "createdate": 1,
    "hp": 1,
    "lastshot": 1,
    "geobinding": 1
}

使用投影查询运行6秒，没有77毫秒

我正在运行mongodb 3.0.3

编辑： 似乎“_id”：投影中的1减慢速度，当我删除它时，它很好，但为什么呢？