My iPhone 3GS backups had become incredibly slow, taking over 10 minutes on a fast computer. The backups and syncs were almost instantaneous when I first plugged in my iPhone, and the problem was vexing because my configuration, apps, and general information hadn’t changed.
I looked on the Internet and found several false leads. The most promising-sounding — but absolutely worthless in real life — was the suggestion to delete the old backups which were probably corrupt. I spent minutes in iTunes discovering where the “delete the backups” function was, and then discovered no speed improvement.
I finally found the answer to solve the slow iPhone backups on the Apple site that worked for me: delete the photographs and videos in the “Camera Roll”. You can download the photos and videos from the iPhone to your PC (copy them using Windows Explorer if you’re on a Windows machine). Then delete them from the iPhone… I did the delete in a Windows Explorer window while my phone was connected. Now the iPhone backs up and syncs in under a minute.
If you’re having problems with slow backups try copying and then deleting your camera roll. Having photos in other folders seems not to affect the speed.
And, I linked above to Apple’s answer to help it rise in Google search results. If I saw the right 2010 answer earlier, I would have saved time and angst!