First query after index creation is slow

I added an index to the collection. The first query I run is slower than the same one without an index. The following are faster than those without an index, so this makes sense.

I'm wondering why this might be happening because the index had to go from disk to memory? Then it's even harder for me to figure out that I drop the index, restart the mangon, I create the index again and it goes fast and not the first time. In case I restart my computer, it behaves like the first time, so it only goes slow the first time with the index.

Can anyone explain this behavior clearly?

Below I will cover information about documents, indexes and queries. The documents inside the collection look like this:

> db.posts.findOne()
{
        "_id" : ObjectId("557d73e1fab73211b00f3080"),
        "title" : "aaa",
        "author" : "nuevo",
        "body" : "aaa",
        "permalink" : "aaa",
        "tags" : [
                "a"
        ],
        "comments" : [ ],
        "date" : ISODate("2015-06-14T12:30:25.733Z")
}

      

        
        
        
      

    

Collection size:

> db.posts.find().count()
1008

      

        
        
        
      

    

Query without index, it takes 3ms (I am not putting the entire output of the explanation, only the relevant parts):

> db.posts.explain("executionStats").find({ permalink: "ambzrbxvnorazgnqvzbw"});

{
....
        "executionStats" : {
                "executionSuccess" : true,
                "nReturned" : 1,
                "executionTimeMillis" : 3,
                "totalKeysExamined" : 0,
                "totalDocsExamined" : 1008,
....
}

      

        
        
        
      

    

Index creation:

> db.posts.createIndex({permalink:1})
{
        "createdCollectionAutomatically" : false,
        "numIndexesBefore" : 3,
        "numIndexesAfter" : 4,
        "ok" : 1
}

      

        
        
        
      

    

Query with generated index (71 ms):

> db.posts.explain("executionStats").find({ permalink: "ambzrbxvnorazgnqvzbw"});

{
....
        "executionStats" : {
                "executionSuccess" : true,
                "nReturned" : 1,
                "executionTimeMillis" : 71,
                "totalKeysExamined" : 1,
                "totalDocsExamined" : 1,
....
}

      

        
        
        
      

    

Restart the same query with a different permalink to avoid it from memory (or something similar). It takes 0ms:

> db.posts.explain("executionStats").find({ permalink: "orrjnueekntvjegzvbjk"});

{
....
        "executionStats" : {
                "executionSuccess" : true,
                "nReturned" : 1,
                "executionTimeMillis" : 0,
                "totalKeysExamined" : 1,
                "totalDocsExamined" : 1,
....
}