drupal 404 page

I have recently monitored a client website in Drupal 7. It was spammed by commenting with spam links. Although I deleted about 600M of spam comments, all of them have been crawled and indexed by search engine bots. At the result, there are quite traffic coming to this site looking for non existing content.

At a peak time, the VPS RAM has been used upto its limit of 2 Gigabyte. The website became very slow. Apparently, responding to those removed spamming content (404 - Page not found) has eaten up alot of the server resource. So I need to find a solution to minimize memory usage for those useless requests.

The problem is, Drupal has expensive 404 errors. When serving a 404 page, Drupal stills do a full "bootstrap", loading all modules, settings, querying d...