I have seen other people reporting that the issue may be related to the option to transmit crash report data back to Apple. I also noticed that after particularly hard crash last week, my backup took considerably longer than normal.
I suppose this could be compounded by your internet speed, and how much crash report data it sends.
In my case, I couldn't even get the phone to shutdown, and ended up having to hold the sleep button down until it powered off. With the next sync, the backup went from 1-2 minutes to almost 10 minutes...
Subsequent backups have been back to 1-2 minutes, even after updating a couple of applications.