The file size of your book can get very large when you have a lot of pictures in it. And when the file is huge it can slow down your computer a lot and make editing drag on and on. If you are struggling with this then consider splitting the book into chapter files where each chapter (or two) are individual files. While you are editing them you can call them whatever you want, but I have a suggestion for when you are ready to take the book to the publisher. By putting numbers at the beginning of the name of each file they will be sorted into the order of how they appear in the book. Use two digits for the numbers (i.e.: 01Titlepage, 02Preface, 03 Chapter one, etc.) to keep the first nine chapters in order on the top of the list.