Mac slow after using "track changes" in Pages?

I will start by saying that Pages is not a Word clone and every time you open or export a Word document, there is a translation to or from Pages internal document format. That leaves room for unknowns in either format.

For years, the recommendation in this community has been to use a current Word app if exchanging Word documents as one can open, edit, and save in the native document format without the vagueness of translation entering the picture. You cannot trust the Finder's Quick Look feature to display the Word document as it would appear in MS Word.

I don't know if you are running any anti-virus, so-called Mac "cleaner/optimizer" applications, or working over a VPN — but if you are, that could be a significant performance deterrent. If Spotlight is indexing, or Time Machine is backing up, you will experience noteworthy performance lag for that duration.

One thing you might do is close and quit all applications. Then reboot your Mac into Safe Boot mode and see if your performance issues remain with this document opened in Pages. If not, then some third-party software installation is causing interference. If it continues, then reboot the Mac normally and try this document in Pages again to see if the problem is transient.

I have a 48,700-word (179 pages) Pages document that opens instantly in Pages v14.3/Sequoia v15.2. I do not use tracking as I have no need to exchange documents with anyone.