Sessions Performance Issue (php7.1, CI 3.1.5)

I just want to point out while the documentation it simply does not work. There are tons of articles and documentation, but a clear answer:

According to CI docs you should do this:
Long story short - call session_write_close() once you no longer need anything to do with session variables.

However, adding this to the Session Lib doesn't work, where are they supposed to go?