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