Client had a Drupal7 site that had the (not yet discovered) bug in that version of Drupal where the views cache was being rebuilt for anonymous users accessing the front page.

While the root cause could be covered up with proxy/cache servers an ultimate solution needed to be found.

One test was to see if replacing MySQL with MariaDB increased speed. It did.  Because the cache was broken in that version of Drupal it was causing excessive DB lookups. Changing from MySQL to MariaDB resulted in a 2x to 7x performance improvement.

Ultimately the solution was discovered to be a bug in Drupal core and views. Upgrading Drupal to the next release solved the issue on both MySQL and MariaDB systems, however moving forward we will be using MariaDB for all future work.