A problem we hear about quite often is a huge cache_form table. Drupal by default caches all form data (typically for 6 hours). During peak traffic times, that can be a lot of form data being cached.
When originally building the Marconi module, I did not realize that Openstack project names change. Most recently, the name of the queue project in Openstack was renamed to Zaqar.
Recently, we were debugging some performance issues with a client's Drupal Commerce website. After doing the standard optimizations, we hooked up New Relic so we could see exactly what else could be trimmed.