Thanks for sending your document. I sent you a direct email reply, but I'll also reply here for completeness:
The problem seems to be related to Split View and Page View. For your document's particular saved state I can alleviate the slowdowns simply by switching the top split view to Draft View. However, you may still trigger slowdowns if you change the exact scrolling or editing locations. To completely eliminate the slowdowns both top and bottom split views need to be changed to Draft View.
I would guess that some particular formatting in your document is triggering unnecessary Page View slowdowns. Perhaps options like "ensure break before paragraph" and "keep with next paragraph" are interacting badly. We'll have to investigate further to see what we can do to improve things.
Thanks again for your help! Please let me know if you have any questions or concerns.