Hi, this is (perfectly) normal : NTFS support under Linux isn't very good. It's done using the ntfs-3g driver, which runs in userspace using the FUSE feature of linux (already some overhead there). Add to that a not completely optimized FS driver, and you get slow speeds. Sorry, ntfs support under Linux at least exists now though...
If I'm wrong, please anyone correct me, this was just my explanation when seeing my CPU usage spike while writing to NTFS.