I had a collection that had two indexes, one ascending, and one descending

mongo> db.url.getIndexes()
{
"_id" : ObjectId("4b1651417fc9cc6b089203f6"),
"ns" : "scan_stats.url",
"key" :

{ "hours.velocity" : 1 }

,
"name" : "hours_velocity_1",
"unique" : false
},
{
"ns" : "scan_stats.url",
"key" :

{ "hours.velocity" : -1 }

,
"name" : "hours.velocity_-1"
}

This query was taking 6 seconds:

Tue Dec 15 01:37:14 query scan_stats.url ntoreturn:1 reslen:224 nscanned:9236896 { hours.velocity:

{ $gt: 0.0 }

} nreturned:1 6934

I removed the unnecessary index (the ascending one), and then the query became instantaneous.