I'm guessing the authors deployed to production with no indexes on their collections. This means the cost of most queries was probably O(n). This could go unnoticed in development where N is small, but is unacceptable once you have any real data. I think they are recommending thinking about and adding indexes while you are developing as opposed to trying to do so post-hoc, once queries are taking tens of seconds.
Comments
Anyone care the explain "2. Create your indexes as you develop." a little deeper.
I'm guessing the authors deployed to production with no indexes on their collections. This means the cost of most queries was probably O(n). This could go unnoticed in development where N is small, but is unacceptable once you have any real data. I think they are recommending thinking about and adding indexes while you are developing as opposed to trying to do so post-hoc, once queries are taking tens of seconds.
MongoDB is basically unusable without indexes even under the slightest load.