Lately my site was slowed down quite often for increased spam activity. Then I installed the Bad Behavior plugin which so far has been doing a great job. It is actually blocking bots before they are able to visit the site and so my server load is greatly reduced.
It works with Akismet, as well. The spam able to pass BB is blocked by Akismet. The only downside is that some legitimate users can be blocked, usually whn browsing from public spaces. I was once refused access to my site while browsing from Dubai Airport Wi-fi.