@swatkats : Painful result with MySQLi. All tested with Cloud Database from Rackspace.
MySQLi : https://www.webpagetest.org/result/121008_57_5TB/1/details/
MySQL : https://www.webpagetest.org/result/121008_BS_5W2/1/details/
Both were run with all default. Technically MySQLi should work better. Pathetically, many links to functions / pages within WordPress WP Admin panel (like Plugin browsing) returned errors after ‘thinking’ for few seconds.
PostgreSQL worked the best almost out of the box.
MySQL with HyperDB :
https://www.webpagetest.org/result/121008_M2_71Z/1/details/
MySQL with HyperDB with WP Super Cache and all tweaks :
https://www.webpagetest.org/result/121008_GP_85X/1/details/
~1 second loading time.