Problems with navigation pane on word 2021 - constantly slowness and freezing

Hello, everyone. My name is Diogo. This is my first post.

I just bought Office 2021 home version today. I've been using 2013 Office version for a long time and decided to upgrade. The main reason was my daily use of a word doc for studying purposes. The doc is long (at least from my perspective), with sth abou 3100 pages and 1.4 million words. With 2013' version, I had basically two issues when using this file specifically

1. Sometimes when I was saving the file and, before the end, it was shutted down without properly save (I guess this one is done with 2021's autosave option).

2. The navigation pane (titles and search specifically words) was incredibly slow sometimes. In fact, it was almost impossible to open the titles and digit the text at the same time, so I had to keep it on the search columm to keep working the text. The search columm, besides that, used to freeze after a search for 10 seconds, max 15 seconds, maybe, but after I could keep the work progress and typing

Well, I bought the 2021 one thinking that would be faster and I would solve this problem, but it is not my experience so far. The overall use is better with this newer version, but it's (much) worse to search words with navigation pane, which is very useful in my daily use of the doc. When the titles panel is open, everytime I digit it freezes a little bit, making harder to use. When I search for a word, it definitely freezes, taking sth about 25, 30 seconds to come back, when it will keep freezing for a while. In simples words: when it comes to use the navigation pane (titles and search, I never use pages miniatures), Word 2021 is, so far, much worse than 2013 Office.

The office keeps freezing sometimes, even not using the navigation pane (sth I was not expecting buying this Office 2021, to be honest), but the situation is worse with navigation pane, like I said.

P.s. I've been editing this file for 18 months now. At some point and for some reason I can't tell, the automatic numbering option (sorry, I use in portuguese and I don't know the name Microsoft uses in english Word version, but it's on the paragraph tab and allows to divide paragraphs in numbers and letters. I guess it's numbered list or sth similar) stopped working, so I started mumbering everything manually. I don't know if this could be interfering.

P.s. 2 Before come here I contacted Microsoft via chatting. He tryed to help me, controlling the PC remotelly and doind some tricks, but at the end he sad it was probably some formatting problem with this precise doc file, so he directed me to his forum

Sorry about the long text and thank you for the help

Diogo