There is definitely a performance advantage in keeping your comment spam to a minimum. If you have a lot of comments, the query time can get pretty out of control.
To make it easier, you should install Akismet if you haven’t already. Akismet will automatically detect spam comments and move them to WordPress spam section. You can then delete all the spam comments in one click, OR you can change the default empty trash days from 30 days to something much sooner.
If so, you’ll want to add the following to your wp-config.php file:
define( 'EMPTY_TRASH_DAYS', 1 );
And if you’re brave and would like to just delete spam comments (and everything else in the trash) without sending them to the trash, you can add the following to your wp-config.php file:
define( 'EMPTY_TRASH_DAYS', 0 );