performance

I have been manually cleaning out the sessions table for some time now. We get about 8000 records per hour and so it does not take long to reach several hundred thousand records. Things start to bog down then. The system becomes very slow, especially if we are displaying the "Who's online" block on the home page, which is something we like to do. Also, the query to delete old session records can take quite a while to perform (up to 18 minutes that I have seen).