SQL databases are like automatic transmission and NoSQL databases are like manual transmission. Once you switch to NoSQL, you become responsible for a lot of work that the system takes care of automatically in a relational database system. Similar to what happens when you pick manual over automatic transmission. Secondly, NoSQL allows you to make more performance out of the system by eliminating a lot of integrity checks done by relational databases from the database tier. Again, this is similar to how you can get more performance out of your car by driving a manual transmission versus an automatic transmission vehicle.
Well, speaking performance wise. From proven usage stats. NoSql is better kf MySQL simply because MySQL creates additional layer under the hood. With NoSql you write everything that you need yourself. No implicit or unnecessary checks. But who is ready for that. And though caching mechanisms are used to better improve MySQL, and because of its popularity, MySQL has plugins and customized features that could enhance it's performance better or as much as NoSql. Should you now change to NoSql, I will suggest not too soon., till it starts getting traction. And that is like an realisable foresight to me.