Atom™ N2800  2c/4t   1.86 GHz+  4Go RAM   80Go SSD  100 Mbps

loc: {
    lon: ,
    lat:
}

var x = mongoose.Schema({
            loc: {
                    lon: Number,
                    lat: Number
            },
            created_at: { type: Date, default: Date.now }
    }, { collection: 'places' });

    xSchema.index({loc: "2d", "created_at": -1});
    xSchema.index({"created_at": -1});

    var Model = mongoose.model('places', xSchema);
        Model.on('index', function (error) {
                if (error) return error;
                console.log("ON INDEX");
        });

ON INDEX
Mongoose: x.ensureIndex({ loc: '2d', created_at: -1 }) { background: true }
Mongoose: x.ensureIndex({ created_at: -1 }) { background: true }

db.places.find({ loc: {  '$geoWithin': { '$box': [ [ 2.1566688432447227, 48.62968892500415 ], [ 2.5523096661210394, 49.07990389337538 ] ], } } }).sort({created_at: -1}).limit(100).explain()

Model.find({
        loc: {
            $geoWithin: {
                    $box:[
                        [ parseFloat(Math.min(values.lngmin, values.lngmax)), parseFloat(Math.min(values.latmin, values.latmax)) ],
                        [ parseFloat(Math.max(values.lngmin, values.lngmax)), parseFloat(Math.max(values.latmin, values.latmax)) ]
                    ]
            }
        }
},function (err, places) {
        if(err){
                callback(err, null);
                return;
        }
        callback(err, places);
}).
sort({ created_at: -1 }).
limit(limit);

"queryPlanner" : {
        "plannerVersion" : 1,
        "namespace" : "test.x",
        "indexFilterSet" : false,
        "parsedQuery" : {
            "loc" : {
                "$geoWithin" : {
                    "$box" : [
                        [
                            2.1566688432447227,
                            48.62968892500415
                        ],
                        [
                            2.5523096661210394,
                            49.07990389337538
                        ]
                    ]
                }
            }
        },
        "winningPlan" : {
            "stage" : "LIMIT",
            "limitAmount" : 100,
            "inputStage" : {
                "stage" : "FETCH",
                "filter" : {
                    "loc" : {
                        "$geoWithin" : {
                            "$box" : [
                                [
                                    2.1566688432447227,
                                    48.62968892500415
                                ],
                                [
                                    2.5523096661210394,
                                    49.07990389337538
                                ]
                            ]
                        }
                    }
                },
                "inputStage" : {
                    "stage" : "IXSCAN",
                    "keyPattern" : {
                        "created_at" : -1
                    },
                    "indexName" : "created_at_-1",
                    "isMultiKey" : false,
                    "isUnique" : false,
                    "isSparse" : false,
                    "isPartial" : false,
                    "indexVersion" : 1,
                    "direction" : "forward",
                    "indexBounds" : {
                        "created_at" : [
                            "[MaxKey, MinKey]"
                        ]
                    }
                }
            }
        },
        "rejectedPlans" : [
            {
                "stage" : "SORT",
                "sortPattern" : {
                    "created_at" : -1
                },
                "limitAmount" : 100,
                "inputStage" : {
                    "stage" : "SORT_KEY_GENERATOR",
                    "inputStage" : {
                        "stage" : "FETCH",
                        "filter" : {
                            "loc" : {
                                "$geoWithin" : {
                                    "$box" : [
                                        [
                                            2.1566688432447227,
                                            48.62968892500415
                                        ],
                                        [
                                            2.5523096661210394,
                                            49.07990389337538
                                        ]
                                    ]
                                }
                            }
                        },
                        "inputStage" : {
                            "stage" : "IXSCAN",
                            "keyPattern" : {
                                "loc" : "2d",
                                "created_at" : -1
                            },
                            "indexName" : "loc_2d_created_at_-1",
                            "isMultiKey" : false,
                            "isUnique" : false,
                            "isSparse" : false,
                            "isPartial" : false,
                            "indexVersion" : 1,
                            "direction" : "forward",
                            "indexBounds" : {
                                "loc" : [
                                    "[BinData(128, C406C10000000000), BinData(128, C406C1FFFFFFFFFF)]",
                                    "[BinData(128, C406C30000000000), BinData(128, C406C3FFFFFFFFFF)]",
                                    "[BinData(128, C406C40000000000), BinData(128, C406C7FFFFFFFFFF)]",
                                    "[BinData(128, C406C90000000000), BinData(128, C406C9FFFFFFFFFF)]",
                                    "[BinData(128, C406CB0000000000), BinData(128, C406CBFFFFFFFFFF)]",
                                    "[BinData(128, C406CC0000000000), BinData(128, C406CFFFFFFFFFFF)]",
                                    "[BinData(128, C406D00000000000), BinData(128, C406D3FFFFFFFFFF)]",
                                    "[BinData(128, C406D40000000000), BinData(128, C406D7FFFFFFFFFF)]",
                                    "[BinData(128, C406D80000000000), BinData(128, C406DBFFFFFFFFFF)]",
                                    "[BinData(128, C406DC0000000000), BinData(128, C406DCFFFFFFFFFF)]",
                                    "[BinData(128, C406DE0000000000), BinData(128, C406DEFFFFFFFFFF)]",
                                    "[BinData(128, C406E10000000000), BinData(128, C406E1FFFFFFFFFF)]",
                                    "[BinData(128, C406E30000000000), BinData(128, C406E3FFFFFFFFFF)]",
                                    "[BinData(128, C406E40000000000), BinData(128, C406E7FFFFFFFFFF)]",
                                    "[BinData(128, C406F00000000000), BinData(128, C406F3FFFFFFFFFF)]",
                                    "[BinData(128, C406F40000000000), BinData(128, C406F7FFFFFFFFFF)]"
                                ],
                                "created_at" : [
                                    "[MaxKey, MinKey]"
                                ]
                            }
                        }
                    }
                }
            }
        ]
    },
    "serverInfo" : {
        "host" : "",
        "port" : ,
        "version" : "3.2.8",
        "gitVersion" : ""
    },
    "ok" : 1