Books are mostly text. The fonts (images of each letter) are built into iOS, and will display very quickly. Imbedded images may affect the book if they are really large, but otherwise book files are relatively are small and easily loaded and displayed.
Most Newsstand magazines are big PDF files with lots of imbedded images. The files are large compared to text files can take a while to be displayed. Because of this the magazine app developers often choose a type of image that will display in stages as it is loaded into memory. What you are seeing is a lower resolution image that progresses to the full resolution over time, usually just a second or two. The idea is that you get something to look at while you wait for the page to finish loading.
