One of the things that a default Drupal installation does is that it handles 404s for static files within Drupal itself. In other words, a 404 for a .css or .jpg file causes a full Drupal bootstrap. This is not necessary, and wastes resources on a site that gets lots of 404s: lots of code is executed, many database queries are performed, only to return a 404 for a static file.
Is your Drupal or Backdrop CMS site slow?
Is it suffering from server resources shortages?
Is it experiencing outages?
Contact us for Drupal or Backdrop CMS Performance Optimization and Tuning Consulting