Why is the "ci_sessions" table type MyISAM?

[eluser]WanWizard[/eluser]
I wonder if that is the real issue.

I have sites with a much higher hit rate, and I notice no delay whatsoever with MyISAM tables. The session table or one of them currently holds over 23700 records, with a gc probability of 10%.

MyISAM doesn't support concurrent write access. If your application uses session variables a lot, the standard CI session class updates the session record every time you modify a session variable. Which means that it is possible that inserts are being queued.
We use a modified sess_write() method, that skips the write. We do an explicit write after the page request has been processed and the output generated, so only one session update per page request. That reduced the number of update queries by over 90%.