Optimize MongoDB Query or Index

Question

I am running a quite ordinary MongoDB query, nothing really complex or special and I am wondering if the time it takes (> 1 sec) is normal or if there's something wrong with my indexes.

I provided an index for this specific query and explain() also tells me it is used, but it makes a full scan of the collection each time and slows down the whole webpage by > 1 sec.

The query:

db.tog_artikel.find({"art_filter":{"$exists":false},"$where":"this._id == this.art_hauptartikelnr"})

Explained:

> db.tog_artikel.find({"art_filter":{"$exists":false},"$where":"this._id == this.art_hauptartikelnr"}).explain()
{
    "cursor" : "BtreeCursor art_filter_1_art_hauptartikelnr_1",
    "nscanned" : 21306,
    "nscannedObjects" : 21306,
    "n" : 21306,
    "millis" : 1180,
    "nYields" : 0,
    "nChunkSkips" : 0,
    "isMultiKey" : false,
    "indexOnly" : false,
    "indexBounds" : {
        "art_filter" : [
            [
                null,
                null
            ]
        ],
        "art_hauptartikelnr" : [
            [
                {
                    "$minElement" : 1
                },
                {
                    "$maxElement" : 1
                }
            ]
        ]
    }
}

The index:

{
   "v": 1,
   "key": {
     "art_filter": 1,
     "art_hauptartikelnr": 1 
  },
   "ns": "togshop.tog_artikel",
   "background": true,
   "name": "art_filter_1_art_hauptartikelnr_1" 
}

Why is the full collection scanned every time? Why is isMultiKey false and how can I optimize this query/index?

Environment is a standalone server, MongoDB 2.0.1, 64-Bit Linux, accessed from PHP w/ php-mongo 1.2.6