Yes, its slower than I would like but its not a big deal. The one thing that IS a big deal, though, is going from portrait to landscape while a book is open. That takes a full 6 seconds on my iPad. Thus, if I am reading, and accidentally tip the ipad too far to the side (I read in bed a lot so there is a lot of adjusting, and this happens), its 12 seconds before I am back in my preferred orientation.
I haven't noticed this problem at all in iBooks.