Have you tried repairing the Office installation (via Windows Control Panel > Programs > Programs & Features > Microsoft Office (version) > Change > Repair)?

Have you tried running Word in safe mode (i.e. hold down the Ctrl key while starting Word)? If that improves things, you most likely have an addin that's responsible for the performance hit.

Have you tried clearing the contents of the temporary storage folder used by Word (typically C:\Users\%Username%\AppData\Local\Temp)? Close all Office Application before doing so.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]