title: Fast n-gram calculation
body: <p>I'm using NLTK to search for n-grams in a corpus but it's taking a very long time in some cases. I've noticed calculating n-grams isn't an uncommon feature in other packages (apparently Haystack has some functionality for it). Does this mean there's a potentially faster way of finding n-grams in my corpus if I abandon NLTK? If so, what can I use to speed things up?</p>