The things about history books is that there aren't (or shouldn't have) any useless part. Every sentence of every chapter is the fruit of months, years or even decades of scientific research. A history book is meant to be a tool for knowledge and for developing one's analysis and perception. This is true for academic history books, not for popularisation works. So yes it's normal to take some time to fully read a history book if your goal is to critically understand the thesis of said book.

It goes without saying that there's a part of History which is art and not science, that is the literary style of each author. Some historians make it so that you can keep reading for hours and hours, other historians have less "novelistic" style which can be harder to hang onto. Nonetheless, that doesn't impact at all the quality of the research.

So overall, it will depend on the book and what objectives you set yourself for your readings. As a tip I would say that you try to read full chapters/paragraphs before thinking that you're stuck somewhere, as sometimes you can find the answer to your problems later in the authors explanation.