Saying PostgreSQL is a better fit in terms of performance is a bit controversial in my opinion, there are quite a lot of tests between the two and most say it is the exact opposite https://blog.michaelckennedy.net/2010/04/29/mongodb-vs-sql-s...
Despite that, can't say for a fact that's the case for Relax since I haven't test both on it. Mongo is behaving really well though, our demo instance has been getting a pound lately (dozens of users at a time) and it's running smoothly even though our machine is not really powerful.
Also in terms of scaling, Mongo also has a great solution for horizontal scaling https://docs.mongodb.org/manual/sharding/
Having this said, not entirely against having different database layers supported in Relax. Since we're using GraphQL would be a matter of creating an abstraction when accessing the data on the queries and mutations resolves. Not on our priorities for now but we're always open for contributions :)
Maybe mongodb will perform better, free of joins, than MySQL/Wordpress/whateverCMS... or maybe the data access pattern will suck and make it slow (and maybe it would be slow with a SQL db too, like other CMSs).
Ideally (in terms of performance), a single K/V request would be needed for each page, and the CMS would handle all the denormalization (and tens of thousands of req/s). I don't know how they've designed their data model...
All in all, a CMS is the most cachable web app ever, as it's more or less meant to replace static pages ;-) The poor performance of existing CMSs has always been hidden by caching layers...