After buckling to the extreme pressures put on me by the noble members of this forum

I have redone my doc and am now doing it page by page. No more such long FN, everything is on top. Does allow me much more freedom to use the margin area. I now fully understand that this was the correct way to do things.
But now something else is driving me crazy: Everything has slowed down to a tortus!
When I open my old DOc, which is over 500 pages long, and hit the PGDN key it flies like lightning. In the new doc which is only 50 pages long now, I hit the PGDN keyt and have to wait till it moves!
What is the word is going on here? Cannot be add-ins or anything like that because both files are running in the same environment.
Is Word taking revenge from me?