We have a company report template in Microsoft Word in which we often collaborate in real-time through Sharepoint / Onedrive on our desktop Word apps. Never really more than about 5 or so people, maximum, at a time.
We've experienced that at some stage or rather document length, the document just becomes so slow to work in that it's not practical to use anymore.

The document has a cover page in which you specify document {Title}, {Author}, and {Manager} as field codes which are then linked to each page's header and footer in which these field codes display. Document title in the header, and author and manager in the footer. (We use the Manager field code for specifying the reviewer of the document). Then there's also a page number on each page, which in essence is also a field code (sorry if I'm using the term field code incorrectly).

I can't imagine that it's purely the document length that's causing the speed issue, because we experience it in documents of less than 50 pages sometimes.
Is the speed / response time issue caused by the field codes on each page, the number of people collaborating, or can Microsoft's servers simply not handle 5 people collaborating on a 50-page document in real-time?