For me personally, one of the most effective means of knocking out the first 95% of spam was using the S25R regex methodology [1] created by Asami Hideo which seems to keep the load on SpamAssassin and ClamAV really low. I've had to adjust the regex rules over the years a little bit but it's really low maintenance for my setup. There are also lists of IP addresses and networks you can block that are known to be malicious which also reduces the load and log volume. [2]
In my experience spamassassin works wonderfully. There are some few false negatives (1-2 mails per week), but I did not have a single false positive which is very important for me. For example Google is much worse in that regard, which forces me to check spam every few days to ensure no legitimate mail ends up in spam, so it's like no spam filtering at all, I have to read it all anyway.
[1] - http://www.gabacho-net.jp/en/anti-spam/anti-spam-system.html [No HTTPS, Sorry]
[2] - https://github.com/firehol/blocklist-ipsets.git