Can you explain what it is you actually do with the content of such a large document? It doesn't sound like Word is the best tool for this scale of content because I'm not sure how you would ever totally avoid the WYSIWYG tendencies that underpin Word. It isn't just holding the text, it is continuously working out where that text sits on a page based on the formatting applied to it. Notepad++ (or a database for that matter) doesn't have the same processing overheads.

If you are considering staying with Word, the suggestions I would have are much like those of Charles and Paul above:
1. Minimise repagination by using either Draft (or possibly Web view)
2. Remove all local/direct formatting by selecting all and pressing Ctrl-Q and Ctrl-Space
3. Turn off all proofing so background spell/grammar is not happening

I'm pretty sure the drastic slowdowns you are seeing are unavoidable in Word so I think if it were me, I'd be looking to other software packages rather than persisting with Word.

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Andrew Lockton
Chrysalis Design, Melbourne Australia