I noticed the same, and I suspect this has more to do with the implementation of the Mongo driver it's using than with Vert.x itself. Vert.x is a very high-performance server.
I thought so too, so I looked at the source. It's communicating with a MongoDB persistor over the event bus rather than querying the database directly. So the poor performance is because there's an extra layer of communication happening, which is kind of misleading for a benchmark like this.
Vert.x was in our original batch of tests (round 1). At the time, its documentation showed that using the persistor and event bus was the standard intended mode of communicating with a database. If someone can point us to documentation showing that is no longer how you're supposed to use a database with Vert.x, we'd be happy to accept a pull request that changes the test. Otherwise, I think it's valid as is, and not misleading.