Remember that Fusion is a Mac application running in OS X that is creating the environment in which the Windows VM can run. OS X is also busy maintaining services, such as networking, which the Windows VM will be using. And then there's all the other things that OS X busies itself with, such as Spotlight indexing, periodic Time Machine backups, etc. If those OS X processes have to compete with Fusion for attention, it'll likely interfere with both Mac and Windows 7 functioning.
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