Lately I've been working to improve the response time of 404 pages on some Drupal 8 sites I help maintain. Depending on the complexity of the site and the status of Drupal's various cache layers, Drupal can be quite slow to generate a full page response, even for a 404 page. These sites I maintain are occasionally subjected to aggressive security penetration scans which generate a ton of 404 responses for completely unique URLs. The scanner can send tens of requests per second like this which can quickly exhaust the resources of a web server.