qmail processes on Smartlist

The Smartlist on my system used to send list mails in batches of
20 at a time, each batch takes about 2-3 seconds, then next batch 
immediately follow.

Recently I notice it *sometimes* take a very long "idle" gap between 
each batch -- about 2-4 minutes, during which the system is not busy,
and qmail is not busy:

qmails  2764     1  0 Jan28 ?  00:00:15 qmail-send
qmaill  2770  2764  0 Jan28 ?  00:00:08 splogger qmail
root    2772  2764  0 Jan28 ?  00:00:01 qmail-lspawn |
     /var/qmail/bin/preline /usr/bin/procmail
qmailr  2773  2764  0 Jan28 ?  00:00:13 qmail-rspawn
qmailq  2774  2764  0 Jan28 ?  00:00:01 qmail-clean

This way, a mail used to take under 5 minutes to distribute now
takes more than half an hour. I wonder what may cause this behavior 
change?

Zhiliang