I am editing a 200 page document. At the 100 page mark, I am noticing long hesitations. (java memory freeing up???)
What is the largest practical document where the page is 80% covered.
Is there a utility to permit me to split the document into roughly two halves. Where the first one is a master document, and the second is the subordinate.
With a 16GiB on your computer, you should not see any hesitations for a doc of only 200 pages…
Maybe it is the way you wrote your document… or your operating system…
As a test I downloaded the .odt here => Writer Guides | LibreOffice Documentation - Your documentation for LibreOffice which is a 448 pages doc with a quite complex formatting (100s of tables, images, and so on…) it is as smooth as a breeze, no hangs or what so ever, all test’s edits AND scrolling down to the end are instantaneous at any pages number 100+, as well as 200+, 300+, 400+…
Tests made on my 4 years old computer with only 8Gb of RAM (Ubuntu-MATE 19.10 / LO 6.4.2) as well as on my laptop (4GB ram / Ubuntu-MATE 18.04 / LO 6.0.7.3) everything is very smooth as expected, I even tried some clicks on the TOC and they all bring me instantaneously to the desired page at whatever triple digit is the page number 
There’s no “largest document for N GB RAM”. But there are problematic layouts that might be CPU-bound (that may lag even with 128 GB RAM, because the problem is not RAM). E.g., if your document is a single tall table. Having hundreds of pages of such a table will inevitably lag, but splitting the table to two (or more) would fix it. There are more examples; you need to find out the problem in your document. You are welcome to file performance bugs that you identify.