Paul Benetis found something interesting on his blog:
After setting it up I started loading it with articles. Everything was gravy until maybe an hour after the initial setup. The blog started to crawl. I mean it was completely unusable. The counter on the bottom clocked the page loads at around 75 seconds. What could be the problem? I started looking for a solution.
That in and of itself isn’t terribly unusual, there are a thousand things that can cause a blog to be slow, but check out what Paul found:
I went ahead and activated Akismet (it took about 5 minutes to navigate to the activation page!!!). Right when I pressed the activate button the blog started acting like any WordPress blog should, fast as hell! Thank you Akismet!
You can read the whole story on his blog. Thanks for sharing Paul!
Akismet 
really? i use akismet and my blog is still slow-ish. might be something i put on it tho. xD
Wow!
That is weird though. How can a plugin like askimet speed up a blog? If anything it should slow it down.
Doesn’t Akismet handle comments after they’ve been submitted to the web log? Meaning the web site ought to still feel the ill-effects of being hammered by spammers, and now additionally converse with Akismet servers while they deem a comment good/evil?
Is there an explanation for why enabling Akismet should speed things up rather than further slow things down?
The author’s explanation doesn’t make any sense to me. How would Akismet be reducing incoming connections? It doesn’t block incoming comments from ever being posted, it just stops them from being publicly displayed until the site admin wants.
If you have the option to discard spam on entries older than 30 days than it can save a lot of DB work because it’s not putting all that crap into your DB.
I couldn’t agree more. Before Akismet, I required people to register in order to post, which meant that I got few comments. After installing Akismet, the comments started flowing in and virtually all of the spam was caught. In addition, I noticed that the blog was performing optimally, something I couldn’t say earlier.