In place updates on documents containing indexed arrays slowing down exponentially with array size

The time needed to update documents containing an indexed array depending on the array size even if the array is not updated or an compound index is affected.

given a document like this:

{

  '_id' : "foo",

  an_indexed_field : 0,

  indexed_array : []

}

and indicies like this

db.foo.ensureIndex({"an_indexed_field" : 1})

db.foo.ensureIndex({"indexed_array.idx" : 1})

Updates (in place and non in place) of an_indexed_field depending on the size of the indexed array. (time for update growths exp)

Note/Workaround
Use old style v:0 indexes instead

see simple js example