Are you reading it using Adobe Digital Editions (ADE) on a PC?
That is not a very good bit of software, but is not usually too slow, there may be something odd about the book.
You could try installing the Sony Reader software on the PC and seeing if that is any faster: I doubt it will be but worth a try.
That software is intended for use in conjuction with Sony eReader devices, but can be installed and used on any PC.