I too am having very slow results with the discussion lists on Mailman. Messages are coming in sporadically. I've had some messages that were mailed at 12:50 pm not come in until 6:41 pm, and another that was sent at 2:11 pm not come in until 8:21 pm.
However, one I sent at 6:48 pm came in at 7:28 pm.
From what I can tell by looking at the headers is that che sent the mail off to frida, and then frida held on to the message for 6 hours before sending it off to the group.
Looks like something's up with frida.dreamhost.com.
(Headers put in chronological order here.)
Received: from smtpoutm.mac.com (smtpoutm.mac.com [184.108.40.206]) by che.dreamhost.com (Postfix) with ESMTP id EA951FE9C for XXX@XXX.com; Thu, 20 Mar 2008 14:11:35 -0700 (PDT)
Received: from che.dreamhost.com (localhost [127.0.0.1]) by che.dreamhost.com (Postfix) with ESMTP id 37D00FE9C; Thu, 20 Mar 2008 14:11:37 -0700 (PDT)
Received: from che.dreamhost.com (che.dreamhost.com [220.127.116.11]) by frida.dreamhost.com (Postfix) with ESMTP id 3FE6616DBED; Thu, 20 Mar 2008 14:11:37 -0700 (PDT)
Received: from frida.dreamhost.com (frida.dreamhost.com [18.104.22.168]) by mac.com (Xserve/smtpin115/MantshX 4.0) with ESMTP id m2L3LQbY024987 for XXX@mac.com; Thu, 20 Mar 2008 20:21:26 -0700 (PDT)
This definitely is a problem at Dreamhost as several different email accounts all had the same delay. (i.e. it's not just a problem with one particular provider holding up an email getting through)