cookie and native session driver deprecated?

Hello,

Sorry if I am asking something repeated, but I couldn't found anything related to this question.

I've never implemented sessions, due to the previous site where I used CI being built upon xenforo, and so, it uses it for any authentication and session.

Im starting another site that I am going to use codeigniter 3.0. I've made a simple search from google and found 3.0-dev documentation without noticing it, and it seemed the native or the cookie driver would make most sense. However, after noticing it was not the final documentation, I went to check the 3.0 documentation, and found that both cookie and native are deprecated.

Is there any big reason why? Storing the session data in both a database or files seems highly ineficient. Am I seeing this wrong? What would be the choice that would give best performance with concurrent users? Or am I viewing this wrong and it will be completely negligible?

Thank you for the help.