I am running Windows Server 2016 (RTM) in VMWare Fusion 8.5.1 on a late 2013 iMac (quad-core i5, 16 GB RAM) with macOS Sierra. At random moments the virtual machine practically grinds to an halt, usually corresponding with high CPU usage for the System process (50%, corresponding to one core). The machine is then extremely slow for several minutes, after which it usually passes (but sometimes it does not and a restart is required).
I am typically running Visual Studio 2015 with IIS Express on the machine while this happens, but I am unsure if there is a relation. Similarly, the issue seems to often appear after I have left the VM idle for a while.
Investigating the System process with Process Explorer shows that the culprit is ntoskrnl.exe!SeAccessCheckEx, which appears to be a routine related to UAC. However, disabling UAC does not help.
The VM has been assigned two cores and 8 GB of RAM.