When I have a long document with all parts showing, it's too slow to use it because it is trying to keep retypesetting the entire thing. So when I have it show only one part, it's fast enough to use.
Why not have it keep a page-break cache, or for papyrus paper type a cache of waypoints, like headings or paragraphs, gotten from typesetting only once through, for up until just before the part that's actually on the screen. Since nothing is changing anywhere off-screen as I type except for after the part that's on screen, it only needs to know the cached location just before the screen display starts in order to format the part that's on the screen now. And everything below the bottom of the screen doesn't need to be retypeset every time I enter a character either.
Of course, the newest Guile with the virtual machine and byte code compiler will also make a huge difference. I wish I had enough free time to learn how to be the one that does this work. Unfortuneatly, I will be too busy at least until next autumn... and don't count on my having enough time to learn enough about it then and to do the work. I'll help if I can. I hope that a very bright student is preparing to take on this work. It will make TeXmacs a lot better!