If your computer is slow, use master documents. Most word processors can make each chapter a separate chapter, then join them together as a book. Moving between chapters will be slower, but saving, typing, etc. will be faster.

I’ve never used Microsoft Word in master document mode. Several people have told me that it's a "bug-ridden monstrosity." If this is true, and you need to make master documents, consider another software application, e.g., Adobe InDesign.