The size of your libraries shouldn't have any real effect on the performance of the Word plugin. The size of the document could have an effect.
Restart Zotero and try again in a new, empty document, and assuming that's fast, try again in your existing document. If you think there's a problem, provide a Debug ID (different from a Report ID) for reproducing the problem.
Restart Zotero and try again in a new, empty document, and assuming that's fast, try again in your existing document. If you think there's a problem, provide a Debug ID (different from a Report ID) for reproducing the problem.