How can I make a document of 100+ pages?

If you are making a large document, especially of 100 or more pages, because of the way Scribus updates the screen, it will become quite slow.

The solution to this is to break your document into chapters or sections of no more than about 30 pages each. This of course leads to the issue of how to splice these multiple PDFs together for your final project.

At present, the only way to do this is with programs or utilities outside of Scribus. Here are some freely available choices.

pdftk
Pdftk is an open-source program which not only will join a number of separate PDFs into one, but also a number of other operations, like splitting a PDF into a number of single-page PDFs. Available for Windows, Linux, MacOS, FreeBSD, and Solaris.

PDFjam
In the PDFjam collection is pdfjoin, which is another option for joining separate PDFs. Available for "Unix-like operating systems", such as Linux, MacOS, Solaris.

Multivalent
Another multitalented program is Multivalent, again freely available. These are Java tools for operating on documents of various types, so it will work with Linux, Windows, MacOs, Solaris, as long as you have Java 2 v1.4.