That's correct, that's what sticky sessions do they send all requests back to the same box. No it's not ideal but it's better than 5s page loads. You could try Redis and see if that helps. If it doesn't I'd wager the performance bottleneck is an issue with your application code and not CI. I've never tried Memcache but nothing inside our application takes 5s to load.
If you use LB managed sticky sessions use the files driver as you would for a single instance. You should never have to change php.ini settings for anything related to sessions while using CI AFAIK.
The downside to this if you deploy a new application bundle that requires an instance restart or if they are killed by managed updates every user who's using sessions on that box will be logged off.
You could also try the database session library.
You should create a separate thread for this though, this thread is about session locks not performance issues.
