Insert some documents

var f = 10
var n = 100000

doc = {}
for (var i=0; i<f; i++)
    doc[i] = i

db.c.drop()
for (var i=0; i<n; i++)
    db.c.insert(doc)

Skip over all but 1 using _id index, get the last:

db.c.find().hint({_id:1}).skip(n-1).next()

Performs about 2x worse in 2.6.4 than 2.4.10:

v 2.4.10 n 100000 f 10 t 49
v 2.6.4 n 100000 f 10 t 93

The reason seems to be that mongod is fetching the skipped documents in 2.6.x but not in 2.4.x, so the ratio could be much worse if the collection doesn't fit in memory and the skipped documents must be paged in.