Performance Drop?

[Max Kipness]

Hi,
 
I've noticed a severe drop in performance as of late. One thing to note
is that when looking through the log there will generally be up to 40
messages being batched together. In the past I remember there just being
a couple. There are two things that I can think of that may be causing
the problem but wanted to confirm.
 
1. Does it effect performance by having too many entries in the
blacklist? Currently I have 950 and it's growing as I've allowed users
to blacklist through an automated mailer script.
 
2. Checking several RBLs via MailScanner, and also checking RBLs via
SpamAssassin. Do I need to only allow one to do them?
 
Other than that is there anything to look for? Again, this just happened
recently.
 
Thanks,
Max