Hey Kohavi. I'd be happy to discuss how we might be able to assist you. I represent TableauConsultants.com (a part of Delve Partners LLC) and we're dedicated to professional help with Tableau for enterprise businesses. Shoot me an email and we can setup a time to chat. You can reach me at Jack.Hoyt@delvepartners.com (I'm on the Tableau site under one of our partners, who had an account setup). Thanks,
I work on our Professional Services team, here at Tableau Software. We have Solution Architects and Business Consultants that can help in several ways. Here are some of services that we offer: http://www.tableausoftware.com/support/consulting. It sounds like you may be able to benefit from a health check.
Click on the link of that page to send us an email. We'd be happy to help.
A good first step (if you haven't already) is to download the Tableau Server Performance Analyzer from InterWorks.
I don't know where you're based but if you would like further help then speaking to one of the Tableau team at InterWorks is bound to be useful as the InterWorks team has lots of experience with extremely large datasets (Facebook, for example). If you think this could be of value to you please go to www.interworks.com for contact details for the US and EU offices.