It's a while since I used that sort of configuration but I don't think it's necessarily out of the question. Defrag speed is dependent on the processor speed, the amount of fragmentation on the disk and disk access speed - I guess there can be an awful lot of data being moved around.
If you haven't defragged the disk before (or for a while) you could probably expect it to take some time - try running defrag again, it should be much faster this time because there's not much defragmentation on the disk.
All this is based on my experience rather than a sound understanding of how Defrag works - so I'll defer to anyone who can give a technically correct answer.