Why might a .NET Framework or Visual Studio update take so long?

This isn't exactly a development question, but it does affect my productivity!

Recently, installing updates to Visual Studio 2010 or .NET 3.5.1 or 4 have taken a very long time, up to 90 minutes per update. Is there any particular reason why, i.e. are there any intensive processes being carried out as part of the update?

Could the following affect the speed?

• Windows 7 Ultimate running within a VMware Fusion virtual machine. I am running a MacBook with a 2.1GHz Intel Core 2 Duo and 4GB RAM. The VM has 2GB RAM assigned and 2 processor cores.
• Is the Native Image Generator (NGen) 开发者_开发问答running as part of the updates? Is its performance affected by running in a VM?

what about the hard disk. is it a virtual hard disk or running a native disk?