Consider upgrading your RAID controller to an H710. The H310 is a low-end controller and is your bottleneck.
Hello, We are experiencing verry poor write performance on our R520 with 2012 R2. For example, installing a 2012 R2 VM is faster on my home computer than on this server. As we wanted to use this server mainly for visualization this is starting to be a real problem. All harddisks are in a single raid 5 array. there is a 200GB partition for OS (C:\). and a 2,6TB partition for data and vm's (D:\). Setup:POWEREDGE R520.PERC H310 RAID CONTROLLERPERFORMANCE BIOS SETTING2x INTEL XEON E5-2430 V2 2.50GHZ4x 16GB RDIMM, 1600MHZ6x 600GB, SAS 6GBPS, 3.5-IN, 15K RPM  [IMAGE][IMAGE]  Thanks in advance, and apologies for my poor English.
Dell R520 Slow harddisk performance with 6x 600GB SAS 15K