Norton Ghost - Speed of operating

Have just started using Ghost, but am concerned about the amount of time it is taking to create a recopvery point. I have two drives which i am trying to protect - "C" is 31GB and takes over 21 hrs to do, and my "D" drive is 16GB and that takes 12 hours.

 

I am using an xternal USB drive, and am wondering if these are realistic times, or should they be much faster?