Backing up is the reason why it is so slow. To speed synchronization up, disable backup. Data is still synchronized, and it is reversible. Do this at your own risk. Instructions are for Mac, run it from the Terminal.

To speed up (disable backup):

defaults write com.apple.iTunes DeviceBackupsDisabled -bool true

To re-enable backup:

defaults write com.apple.iTunes DeviceBackupsDisabled -bool false

You probably need to restart iTunes for this to take effect.