The number of reference documents may a significant issue - the document I tested on has 142 - but only if you're trying to update that and the citations at the same time.

FWIW, you'd probably get better update performance if your Bibliography used a Bibliography field instead of using Word's Bibliography building block. Tables of Contents built using Word's TOC building block are well known for their performance hit and I suspect the Bibliography building block is no better. You could also replace the Citation building blocks with Citation fields, but you'd then need to learn the field coding to edit them.

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