When a relational database has better performance than a no relational When a relational database, like MySQL, has better performance than a no relational, like MongoDB?

I saw a question on Quora other day, about why Quora still uses MySQL as their backend, and that their performance is still good.
It depends on your data and what you're doing with it. For example, if the processing you have to do requires transactions to synchronize across nodes, it will likely be faster to use transactions implemented in an RDBMS rather than implementing it yourself on top of NoSQL databases which don't support it natively.