DBSession becoming slow

[eluser]ACSparks[/eluser]
We use the DBSession library to manage the user session and extra data in our database in the ci_sessions table. When the table grows to around 60 thousand rows we see random slow queries when performing INSERT and UPDATE statements on the ci_sessions table. These slow queries also show up in the MySQL slow query log.

What can we do to improve performance of this table? Keep the entire table in memory? Unfortunately, the slow queries do not happen on every page load, and we haven't been able to figure out a pattern yet.

We also tried changing from an InnoDB table to MyISAM, but that didn't seem to change anything

Any insight or suggestions would be much appreciated! Thanks!