I think it was posted a while back; it was an interesting enough article that it stuck in my head and I was able to find it pretty easily by googling "why is grep so fast?".

Ah, here it is:

https://news.ycombinator.com/item?id=2393587

And again!

https://news.ycombinator.com/item?id=6813937

And duplicate submissions are listed in those postings.