The greatest performance difference for me came when I turned off remote checking. By default, SA will connect to other servers on the Net which maintain a database of spammers. If the email is from an address in the database, SA will mark the message as spam.

For my needs, I found the delay in processing email could really cause problems. If someone sent even a legitimate email to 10 recipients, processing could take longer than the spamc timeout.

I disabled remote checks by starting spamd with the -L flag and did not notice a decline in spam catching ability; however spam filtering processing time (wall time) dropped from about 7-8 seconds to less than 1 second.

SUMMARY

To speed up processing, use spamd and spamc to filter your mail through SpamAssassin.

The exact command line I use to start spamd: spamd -d -L -S