We have optimized core queries to work with few nscanned and low ms. This optimization works in 2.6.11 and 2.4.9.

However, in 3.2.3 performance regressed badly.

In 2.6.11 we have,
"nscanned" : NumberInt(11),
"nscannedObjects" : NumberInt(10),
"keyUpdates" : NumberInt(0),
"numYield" : NumberInt(0),
"millis" : NumberInt(0)

In 3.2.3 we have,
"keysExamined" : NumberInt(10762703),
"docsExamined" : NumberInt(2208753),
"numYield" : NumberInt(84084),
"millis" : NumberInt(33272),

See the query, indexes, collection statistics and full profiler input in the attached document.

Note that the documents in the PostKey2 collection are similar to the documents described in SERVER-22872.