Apparently AHBL_RHSBL has been performing very poorly, detecting 0.072% spam during the August 2009 rescore masscheck and 0.02% spam in recent masschecks. This is not worth a DNS query for every mail you scan. Well, this rule is not harmful, but you may want to disable it if you want a little more efficiency. Insert this line below into your local.cf and restart your spamd daemon.
score DNS_FROM_AHBL_RHSBL 0