How can we improve CI 3 sessions?

I have no idea, I guess it doesn't take up too much space being a very small file size.

The only other thing I could think of was running a cron job periodically during the night to purge the session folder, but what is night time for anybody in the world. It's different?

Sessions already have their own garbage collection, AKA "gc". Yes, you can do your own garbage collection, but you would ideally be using database sessions, because that allows you to purge by datetime.

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