Considering most CMS systems barely reach 100req/s (uncached), I think we can say raw database speed is less of a problem than bad query patterns. Or if you prefer, the use of a SQL db is no guarantee for performance.

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...