Let's take for example a blog post where a unique slug is generated from the post's title: sample_blog_post. Instead of storing a mongo ObjectId as the _id, say you store the slug in the _id. Besides the obvious case where the slug may change if the title changes, are there major disadvantages in terms of performance by using a string instead of a numerical _id? This could become problematic if, say, the number of posts became very large, say, over a million. But if the number of posts was relatively low, say, 2000, would it make much of a difference? So far the only thing about the ObjectId that I think I'd take advantage of is the created_on date the comes for free.
So in summation, is it worth it to store the slug as the _id and not use an ObjectId? There seems to be discussion on how to store alternate values as an _id, but not the performance advantages/disadvantages to it.