CI3 Database Session performance problems

Hi guys, 
I just found that all the performance problems i had with my application were caused by sessions stored in the database.

I know that the session library has been rewritten in CI3 and i was using database sessions before on CI2 without having this kind of problems. Basically when I upgraded i started having performance issues, the app was super slow even while doing basic queries. 

It was a good thing at the end, because i started to dig down my application and database in order to optimize and cache everything i could, but then today i discovered that switching from db sessions to file sessions everything goes super fast and smooth! 

I'm happy that i figured that out, but at the same time i want to rise this topic up in order to find out if you have experience something similar and potentially avoid problems like that on your apps.

Maybe there was something else wrong, i don't know if database sessions are not recommended for applications that have some hundred of users connected at the same time, doing heavy queries both with web and mobile apps. My ci_session table had thousand of rows, iand maybe just adding an index might have helped. 

I think it's worth to know if there's a problem in how database sessions are implemented.

Thank you.
Valerio