Symptoms
When you install an update for the Microsoft .NET Framework 4, the Native Image Generator (NGen.exe) uses a high percentage of the CPU cycles on the computer for a long time. This time varies with how many Native Images are installed on the computer.
Cause
Updates to the .NET Framework 4 require a complete regeneration of the Native Image Cache, a very time-consuming operation. For some computers, an interaction with previously installed Native Images may cause Native Image regeneration to take much longer than expected. Although this issue only affects setup times, the effect can be several minutes to tens of minutes. Computers that have more Native Images installed will see longer generation times.
Workaround
To work around this problem, you may be able to reduce Native Image Generation times somewhat by forcing NGen.exe to generate new Native Images without regard for what was previously generated.
To improve regeneration times, immediately before you install the .NET Framework 4 update, open an elevated command prompt and run the following commands.
Important Run these commands immediately before you apply the update. Otherwise, these commands are less effective.
On a computer that is running a 32-bit Windows operating system
%windir%\microsoft.net\framework\v4.0.30319\ngen.exe update /force /queue
On a computer that is running a 64-bit Windows operating system
%windir%\microsoft.net\framework\v4.0.30319\ngen.exe update /force /queue
%windir%\microsoft.net\framework64\v4.0.30319\ngen.exe update /force /queue
