This happens currently once a week, someone calls me and say “I’m using Storage Spaces with or without Scale Out file server and the performance is #”"!&%%&##”

There is many reasons but a very common is that MPIO be default is set to Round Robin and that is worse then bad.

Change the setting for all disks and for new disks as well by running the following command from en elevated PowerShell prompt:

Set-MSDSMGlobalDefaultLoadBalancePolicy LB

Read more here: http://support.microsoft.com/kb/2744261

Also read the blog from Jose:

http://blogs.technet.com/b/josebda/archive/2013/04/17/file-server-tip-how-to-rebalance-a-scale-out-file-server-using-a-little-powershell.aspx

http://blogs.technet.com/b/josebda/archive/2014/04/01/step-by-step-for-mirrored-storage-spaces-resiliency-using-powershell.aspx

http://blogs.technet.com/b/josebda/archive/2013/10/30/automatic-smb-scale-out-rebalancing-in-windows-server-2012-r2.aspx