Creating index takes very long time

I created a collection in MongoDB consisting of 11446615 documents.

Each document has the following form:

{ 
 "_id" : ObjectId("4e03dec7c3c365f574820835"), 
 "httpReferer" : "http://www.somewebsite.pl/art.php?id=13321&b=1", 
 "words" : ["SEX", "DRUGS", "ROCKNROLL", "WHATEVER"],     
 "howMany" : 3 
}

httpReferer: just an url

words: words parsed from the url above. Size of the list is between 15 and 90.

I am planning to use this database to obtain list of webpages which have similar content.

I 'll by querying this collection using words field so I created (or rather started creating) index on this field:

db.my_coll.ensureIndex({words: 1})

I started creating index about 3 hours ago and it doesn't seem like it could finish in another 3 hours.

How can I increase speed of indexing? Or maybe I should use completely another approach to this problem? Any ideas are welcome :)