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

1. Disable Automatic Navigation Pane Updates 
Open the Navigation Pane (Ctrl + F or View > Navigation Pane).
Click the ▾ drop-down menu in the upper-right corner of the Navigation Pane and uncheck “Auto Update” (to prevent real-time refreshes from slowing things down).
2. Turn off Hardware Acceleration 
Go to File > Options > Advanced.
In the Display section, check “Disable hardware graphics acceleration”.
Restart Word. 
3. Start in Safe Mode 
Close Word, press Win + R and type: 
shell 
winword /safe 
Open the document in Safe Mode and check if the navigation pane is still lagging: 
If it is normal → the problem is caused by a plug-in, disable the plug-ins one by one to troubleshoot the problem (File > Options > Add-ons).
If still stuck → problem with the document itself or with the Word installation.
4. Rebuild the Word configuration file 
Close Word, press Win + R and type: 
shell 
%APPDATA%\Microsoft\Templates 
Delete or rename the Normal.dotm file (reset the default template).
Re-open Word. 
5. Simplify document structure 
Split documents: Split long documents into sub-documents (merge via Insert > Objects > Text in File).
Reduce the number of styles: Avoid misuse of heading styles (e.g. too many nested headings 1.1.1.1).
Convert to plain text: copy the content to a new document and use Paste Options > Text Only.
6. Update/Fix Office 
Go to Control Panel > Programs and Features and select Microsoft 365 > Changes.
Select Quick Fix > Repair and restart your computer when finished.
7. Check Event Viewer: 
Press Win + X > Event Viewer to see if Word crashes are logged in Windows Logs > Applications.
8. Disable background programs: 
Temporarily disable antivirus, OneDrive synchronization, etc.