Slow query, extra field that the whole collection shares same value

gistfile1.txt

> db.sittings.find({ "user.id" : "504fffe45c69233171000000", "status_id" : { "$in" : [ 1, 2 ] }, "app" : "schools", "metadata.client_id" : "4" }).explain()
{
        "cursor" : "BtreeCursor user.id_1",
        "nscanned" : 4,
        "nscannedObjects" : 4,
        "n" : 4,
        "millis" : 686,
        "nYields" : 0,
        "nChunkSkips" : 0,
        "isMultiKey" : false,
        "indexOnly" : false,
        "indexBounds" : {
                "user.id" : [
                        [
                                "504fffe45c69233171000000",
                                "504fffe45c69233171000000"
                        ]
                ]
        }
}
> db.sittings.find({ "user.id" : "504fffe45c69233171000000", "status_id" : { "$in" : [ 1, 2 ] }, "metadata.client_id" : "4" }).explain()
{
        "cursor" : "BtreeCursor user.id_1",
        "nscanned" : 4,
        "nscannedObjects" : 4,
        "n" : 4,
        "millis" : 0,
        "nYields" : 0,
        "nChunkSkips" : 0,
        "isMultiKey" : false,
        "indexOnly" : false,
        "indexBounds" : {
                "user.id" : [
                        [
                                "504fffe45c69233171000000",
                                "504fffe45c69233171000000"
                        ]
                ]
        }
}