I would check the log file to see how big the backup is and what it is backing up. A friend of mine had a similar issue - turns out, he was backing up some really huge temp files that weren't required to be backed up - he excluded those files in the exclude list and voila - a much faster backup!
Also, I heard the rumor that if you have Parallels installed and you haven't excluded the Parallels drive from the backup, then merely launching Parallels is enough to tell Time Machine to backup the entire virtual drive - even if you made no purposeful file changes to the virtual drive.