The WordPress Anti-Spam-Plugin Akismet features the option to show the number of approved comments for each comment author in the comments overview.
While this is a somewhat interesting feature to see if this is a regular user commenting, it significantly slows down the admin interface on blogs with a large comment database. This can be fixed by adding a new index to the database assisting the underlying query Akismet does:
CREATE INDEX akismet_comment_count_loggedout ON $PREFIX_comments (comment_author_email, comment_author(100), comment_author_url(100));
Please note that you have to replace $PREFIX with your table prefix, obviously.