Strategy: Caching 404s Saved the Onion 66% on Server Time

In the article The  Onion Uses Django, And Why It Matters To Us, a lot of interesting points are made about their ambitious infrastructure move from Drupal/PHP to Django/Python: the move wasn't that hard, it just took time and work because of their previous experience moving the A.V. Club website; churn in core framework APIs make it more attractive to move than stay; supporting the structure of older versions of the site is an unsolved problem; the built-in Django admin saved a lot of work; group development is easier with "fewer specialized or hacked together pieces"; they use IRC for distributed development; sphinx for full-text search; nginx is the media server and reverse proxy; haproxy made the launch process a 5  second procedure; capistrano for deployment; clean component separation makes moving easier; Git for version control; ORM with complicated querysets is a performance problem; memcached for caching rendered pages; the CDN checks for updates every 10 minutes; videos, articles, images,  404 pages are all served by a CDN.

But the most surprising point had to be:

And the biggest performance boost of all: caching 404s and sending  Cache-Control headers to the CDN on 404.  Upwards of 66% of our server  time is spent on serving 404s from spiders crawling invalid urls and  from urls that exist out in the wild from 6-10 years ago.  [Edit: We  dropped our outgoing bandwidth by about 66% and our load average on our  web server cluster by about 50% after implementing that change].
A minority of our links are from old content that I no longer know  the urls for.  We redirect everything from 5 years ago and up.  Stuff  originally published 6-10 years ago could potentially be redirected, but  none of it came from a database and was all static HTML in its initial  incarnation and redirects weren't maintained before I started working  for The Onion.
Spiders make up the vast majority of my 404s.  They request URIs that  simply are not present in our markup. I can't fix a broken spider and tell it not to request these links that  do not even exist, but I still have to serve their 404s.
Our 404 pages were not  cached by the CDN.  Allowing them to be cached reduced the origin  penetration rate substantially enough to amount to a 66% reduction in  outgoing bandwidth over uncached 404s.

No joke, irony, or sarcasm intended. Most of this traffic is from spiders looking for made up pages so preserving URLs isn't the issue. The issue was reducing the impact of these poisonous spiders and caching the 404 page was the antidote. Even if you haven't been on the web for over decade like The Onion, there may be a big within is easy reach.