I notice that iBooks rather frequently hangs when scrolling through the text while reading. I am in night mode and the font is at the smallest possible size, it is AFeastforCrows bought on iTunes, so there is a lot of text visible on the screen and every 4 or 5 pages or so there is a small "hiccup" where it suddenly hangs while slowly scrolling... then continues. That's on my iPad3. It is there on the iPhone5s too but the hang is much shorter, so it's barely noticeable but it is there.
All of a sudden it has become a lot harder to swipe-up the system controls or swipe-down the notifications overview while reading, too.