Why is my Digital Editions 4,5 so slow?

Hi, 

Why is digital editions 4,5 sluggishly slow when scrolling through a book? When the book is set to "Fit page" the text is too small to read but not as slow to scroll. However, when changed to an adequate size for reading eg. "fit width", the scrolling is so frustratingly slow I can barely force myself to read the text. The app just keeps loading and loading. I need help in how to fix this.