>for example you'd need to make multiple queries of different tables: first the "article" table to get the text, then the "author" table, then all the comments, tags, images, or whatever
Haven't you heard of joins?
Haven't you heard of joins?
Where it becomes extremely impactful is when a database is distributed or sharded across a cluster. If the various tables (article, author, comment) are stored on different servers, it takes time for the database to reassemble them. Moreover, modern web apps often have content that's written once, read many times: blog posts, photo posts, status messages, news articles, advertisements, catalog listings, etc. Why re-do that work of assembling the content every time people want to look at it? When documents are rarely or never edited, the relational model's defenses against "anomalies" become less relevant. Far better to use a document store like MongoDB, which stores a piece of content as an integrated whole at a single place in the database.