Hi Diitri:
Oh, you can flat-line the CPU with a ONE LINE document if there is something wrong with the document :-)
The size of a Word document has little to do with any problems, the complexity of the document is the key. Can you tell us what is in the document? Be suspicious of things like text boxes, tables, tracked changes, comments. These can get mutilated, particularly
if the document was not created in .docx and cause all manner of problems.
In your case, my first suspicion would be the Spelling and Grammar checker. Turn OFF spelling and grammar, and see if that makes any difference.
Go to Word>Preferences>Spelling and Grammar and turn OFF "Check spelling while you type", "Check grammar while you type" and "Check grammar with spelling". Save, Close, and Quit Word.
Has anything changed?
If so: select some text, and go to Tools>Language>Set Language and tell us which language is highlighted. Try this several times, because a document usually has more than one spelling language in it.
Tell us which languages you find: some languages have a few issues like this: European languages come to mind.
Basically, the Spelling and Grammar engine is one of the few components of Word that can peg the CPU without crashing the app. Lotsa tracked changes can do it. Corrupted graphics maybe. But a problematic spelling language is the most likely.
Let us know how you go...