Customers who are installing Exchange cumulative updates may notice an increased install time of 50% or more. There are two common causes of this behavior.
Antivirus software running on the server can drastically slow down the cumulative update installation process. If you are seeing very slow installation performance and are at risk of exceeding your outage window, consider stopping your antivirus software until the installation has completed.
The other common issue is Windows Server 2012 R2 systems that have applied Windows Update KB3097966, an update that removed trust for four code signing certificates that were inadvertently disclosed.
Before running Exchange updates it is recommended to run the following command on the server:
C:\> %windir%Microsoft.NETFramework64v4.0.30319ngen.exe update
C:\> echo %errorlevel%
You only need to perform this step once per server. It can be performed as a preparation step before your cumulative update installation. There is no restart prompt after running the command, nor does one appear to be required.