A quick follow-up to my post from the other day, Making Drupal faster, which was a followup to my post from a month or so back, Drupal Optimization:

After disabling user-avatars at Grown Up Geek, CPU usage has plummeted from daily averages of 35% – 55% down to an amazing 11%-13%. I can say with confidence that this single, simple change has made the greatest impact on site performance since moving from a shared host to our dedicated server.

If your Drupal site is running slow and you have user-avatars enabled, try disabling this feature to see if you get the same astounding results that I did.