Performance issues while sync OLAP DB

Hi,I'm syncing OLAP DB every day from the backend server which process the cube to the frontend.The cube size is 100 Giga.The problem is that sometimes the sync time is 30 min and sometimes 2 hours.Can any body help me understanding the difference?In addition to that, is there any best practices document for SSAS (2008 R2)?Thanks,Ran.