I am unsure whether the following suggestions will improve the situation significantly but they should.


Managing Large Word Files and Combining Documents Into One File In Microsoft Word
If you are not using styles for your formatting, that significantly slows down a longer document. See The Importance of Styles in Microsoft Word.

How many footnotes do you have?
What does "several seconds" mean? Can you quantify this?

The document is 374 pages and has some 2800 footnotes.
It is a doctoral thesis.

It takes between 4 and 6 seconds from clicking "Insert Footnote" to the point where footnote text can be entered.