When I click on a citation and then when the little box comes up for it (which if it does at all takes a long time), and then I click 'suppress author' and enter, then I get the red ribbon without the blue line that marks time/progress. Usually it just seems to freeze there. Once, after waiting almost a half hour, it did make the change. Since the document had been under an older Word (which I updated), it reads 'compatibility mode.' I copy/pasted to a new Word doc and Zotero still having problems--the citation eventually did update after a very long time, but, e.g., the blue line is not showing up in the red ribbon after clicking enter, and it's hard to know when Zotero is stalled or just taking a long time, as it exceeds 20 minutes before I try to restart.

I'm afraid you're just hitting speed limits -- Zotero is particularly slow in Word for Mac and an 800 page doc, presumably with a commensurate number of citations, is just not going to work particularly well.

If at all possible, we'd recommend editing in sections such as individual chapters. If that's not at all possible, LibreOffice on any operating system of Word for Windows are much faster.