Relational tables are written to disk in pages. If I have to join 100 tables to get one or a few records from each, that means I have to read from many pages on disk, and this can be slow to very slow depending on how fragmented your pages are across the disk (and of course how well you set up indexes relative to the particular query plan you are running).
In a document database, all the data for a document is stored together. So if there are N documents, in theory a document database would be around N times faster retrieving all the data for a single document.
And "30x" sounds like a magic number. I'm not disputing it since I don't know where it comes from, but it's probably either for an average query plan or the average across all common query plans or something else that isn't specific enough to have any idea how it would fare for a particular type of query.