System performance might have been slow due to Office updating (MS released a swag of updates on 17 October) or because your AV software was doing a scan.

If the document is still sluggish, it might have acquired some form of corruption. Corrupt documents can often be 'repaired' by inserting a new, empty, paragraph at the very end, copying everything except that new paragraph to a new document based on the same template (headers & footers may need to be copied separately), closing the old document and saving the new one over it.

Other than that, it might be worthwhile repairing the Office installation (via Programs & Features > Microsoft Office > Change in the Windows Control Panel).

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]