problem with ctrl pg up and down

I have a file of about 16 mb, a single page drawing of fields of flowers etc, broken into layers and its fine but my computer is really laboring to carry out the simple step of "ctrl page up or ctrl page down!" if I want to move an object to the bottom or top by entering "shift page up" or "shift page down" it's not a problem but if I try moving an object in increments to the bottom or top the computer has a hard time doing it. Funny thing is that I tried the same thing on some other multi-page files of 100 mb or so and there's no problem! it's a bit spooky, hoping someone can shed some light here

Merry Xmas