Please visit the following file in the source code of WP-Ban:
https://plugins.trac.wordpress.org/browser/wp-ban/trunk/wp-ban.php
Scroll down to line 181.
See where it calls gethostbyaddr()
That does a DNS lookup on EVERY visit to your site if the IP has not already been banned. It will slow your site down to a crawl and you won't see an increase in CPU disk or memory usage and you'll wonder why.
That is why we don't block by hostname.
Instead we offer blocking by IP range and give you a way to look up the IP range of a particular hosting provider using the WHOIS function built into Wordfence and the integration lets you do this with a few clicks.
To find out more about how gethostbyaddr() slows down your site see the PHP documentation here:
http://www.php.net/manual/en/function.gethostbyaddr.php (do a search for the word 'slow' without quotes)
And here's a google search:
https://www.google.com/search?q=gethostbyaddr+slow
Now do me a favor and go out and spread the word that gethostbyaddr() is VERY BAD™ and anyone using it on a production website should be tarred, feathered and woken up at 4am to the sound of The Scorpions "Rock you like a hurricane" blaring through cheap headphones.
Regards,
Mark.