Quoting myself from the first link:

I just had occasion to edit a 100-page document that
was created without using styles. It was formatted completely with
direct formatting. Each page ended with a page break.

Each time it had to be saved, the save took more than 60
seconds, during which time Word was frozen.

A similar-sized document formatted using Styles takes less than 3 seconds for me to save.
When a page break must be forced, instead of using a manual page
break, the paragraph to begin the new page is formatted using a
style that has "page break before" paragraph formatting.

This difference was due entirely to the document being directly
formatted - a much higher level of complexity. Each paragraph mark
in a directly-formatted paragraph carries with it up to fifty
different formatting commands. When formatted using a style, that
paragraph mark will carry with it one command - use this style!