For driving SP 2013 with only 12 GB of RAM I recommend that you stop the search from doing continous crawls, and to [stop][1] any services that you do not plan to use. Otherwise you will ned a bit more memory.

The search continous crawl uses a massive amount of system resources, so in a low performance environment, it should be turned of if it is not extremely necessary for your purposes. 

Also make sure that the virtual machine is allowed to use more than one processor core in your VM environment settings (This got me once in Hyper-V)


  [1]: http://www.gknzcfc.net/subsite/continuous-crawl-on-sharepoint-2013.html