CiviCRM Wordpress Optimization
Have you tried running mod_php as opposed to fastcgi mode?

In the past when we've had this with our servers our first point of call is to determine which part of the stack is limited to 1 core. Our experience has been that its normally mysql. Could you load up mysql with a few concurrent large DB operations and see if it uses multiple cores? This should then good you a direction to head in i.e. which part of the LAMP stack is being limited.