@adomasven may be able to say more, but do you know if the performance issues are specific to the beta or happen in 5.0.34.4 with 16.9?

For comparison, here is a log to load the Quick Format window in 5.0.34.4 in a document with 1200 Zotero citations:
D553872504

D739106385 was the same document under the beta

New beta version is out, this should be fixed. Do post if you find other issues

Still getting a slightly different index error with current beta:
D1636780388

It does not occur in a fresh document. Here is an example of adding one citation, adding a bibliography, then editing the bibliography.
D650639063

(Note that the bibliography is inserted as a bookmark, not a field, and is formatted as the Normal Word style, not the Bibliography Word style. This is a bug related to Word 16.9.1, not the beta [it's present in Zotero 5.0.34.4])

In a medium length Document B formatted using Bookmarks, not Fields, I am unable to change back to Fields. I receive the error "type not defined":
D909001211

If I delete the bibliography from the document, converting Bookmarks to Fields works, but each citation returns the manual modification warning dialogue, even though none of them have been modified (the citation text for each citation is initially the placeholder text of a date/time field with all zeroes; perhaps Zotero is getting stuck comparing that to the real intended text?). Several citations were also for some reason skipped over and converted to plain text (unlinked) rather than to Zotero Fields.
D819402598

After converting to Fields, the bibliography inserts as normal (though the formatting is incorrect as noted above):
D1622137056

In a previous version of the document already using Fields, I get the error "this.registry.citationreg.citationById[c[0]] is undefined" when I try to add a new citation:
D1742933281

Here is the same error in a short Document C:
D340769956

If I copy all of the text to a new document (including citations and bibliography), the error persists. If I copy everything but the bibliography, the error goes away (same if I delete the bibliography from the original document). So this appears to be an error related to corrupt bibliographies. It's happened now in four different documents with bibliographies generated before the beta integration update (all that I've tested). So it seems like a widespread issue with handling pre-update bibliographies.

In a very long (1200 citations) Document D, doing any operation is still extremely slow (>20 minutes before the citation dialog window opens to Add Citation), and I don't have the time right now to let it run.

Okay, got Zotero to be on 5.0.11 for the Word add-in.

The type is not defined error persists for the bookmarks version of Document B:
D136823413

So does the "citation modified" errors when I delete the bibliography, then switch Fields to Bookmarks:
D243177805

In Document C, still get an "this.citationsByIndex[idx] is undefined" error:
D1745588906

But not getting it or any other registry/index errors in the Fields-version of Document B:
D924819363

Or in another document that had previously given the error:
D1886322349

The first speed issue is fixed in Document D (citation window pops up within a few seconds). However, when I try to add a new citation (remember 1200 existing ones and a bibliography), it has taken >20 minutes to update and the "delay citations" window has not appeared (needed to stop and move the computer).
D709168377

After 20 minutes, the update hadn’t finished and I didnt get the delay citations message. Are you saying that the message wouldn’t appear until after it had _finished_ the first very long update?

The reported issues should be addressed in the latest beta. Thanks for reporting!

No code changes regarding processing speed, so likely coincidental.