Does the VM environment use reserved resources or pooled resources?
Have you tried installing a instance of Tableau Desktop on the same VM machine running Tableau Server? If Tableau Desktop performs slow in the machine running Tableau Server then it has to do with the environment.
Are you running other apps on the same VM? It may be fighting for resources with Tableau Server. Also, anti-virus software could bring Tableau Server down to its knees. Check out the attached VM tuning and best practices pdf and this KB article: http://kb.tableau.com/articles/howto/improving-performance-by-using-antivirus-exclusions
VM_Tuning_and_Best_Practices.pdf 579.1 KB
How are your cores allocated?