You're absolutely right - there's nothing stopping a standard MySQL setup pulling off the same stats, but it just wasn't what we were looking for, coupled with the various other decision points made in the post. RethinkDB's native ability to shard and scale is the key performance case to be made, in my opinion.

As mentioned by another user though, the filtering here is done entirely server-side (at RethinkDB's level), so the V8 thread performance isn't really a concern. https://news.ycombinator.com/item?id=9411738 for more info.