how to optimize SESSION on codeigniter ?

Hc-innov is right. Read this page carefully! It clearly says to NOT remove the lock and what to do instead:

you may conclude that locking is the issue and therefore look into how to remove the locks …

DO NOT DO THAT! Removing locks would be wrong and it will cause you more problems!

Locking is not the issue, it is a solution.
...
Long story short - call session_write_close() once you no longer need anything to do with session variables.

CodeIgniter 4 tutorials (EN/FR) - https://includebeer.com
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