On a recent project a customer notified us a calcution was taking 5 times longer in the new Citrix xenapp 7.6 environment then on a regular workstation.
After days of narrowing down the cause. We found the slow down became apparent after installing the server 2012 RDS role. Since Server 2008 r2 Microsoft has added a feature called fair-share.
In server 2012 fair-share is by default also enabled for disk en network.
More info on fair-share here.
While the cpu setting can be disabled via GPO. The disk and networking setting can only be disabled by registry. In our case the problem was resolved by disabling the disk setting.
Credits to my colleague Sjoerd Elzinga. My partner in crime in troubleshooting this issue.