To keep the loading speed of your php pages relatively fast, and you get a lot of different users/auths in your statistics, you might want to think about cleaning up in the statistics. Below are a list of common waste that builds up in the MySQL database, and solutions.
In the MySQL statements you can usually just replace DELETE with SELECT * to see which rows will be deleted before doing it.

• Most people want statistics because they are a relatively static amount of users that are active in the channel. They do not care much about average joe, who joins the channel, says a few lines, then parts again. That's why it can help a lot to remove all users with below a set amount of lines logged.

  DELETE FROM `stats_users` WHERE `channel`='#channel' AND `lines`<6;

  Deletes all user-statistics from #channel where the user has said less than 6 lines.

• One of the downsides to logging all URLs said, is that you get a lot of trojans or advertisement aswell.

  DELETE FROM `stats_urls` WHERE `url` LIKE '%funstuff%';

  Deletes all URL entries that contain funstuff.
  Keep in mind that only the URL is saved, not the other text said with it.