How fast is the port on your computer? That may be the bottleneck or part of it, especially if you are running FW 400 or USB 2.x. Also, other devices on that interface can slow things down. Just because you have a port that's labeled as such, seldom will it perform at the full spec of the standard. Also look into the speeds of your drives. The drives you have may be 5400 and that could be part of the slowness. The size and quantities of the files you are backing up can also be a factor. If the drive is NAS, that's probably the main issue with speed.
If you can upgrade your computer, consider newer interfaces, such as eSata, USB 3, FW800. USB 3 and eSata will probably get you the biggest speed boost for the $. Thunderbolt, even though it rates faster and seems easy to configure is just so expensive for the drive boxes, IMO.
For my own system, I'm running 2-3GB 7200 rpm drives in an external eSata box with a port-multiplier inside my Macpro. No complaints with speed and I'm backing to two external drives in two JBOD boxes so I have redundancy (knuckle-head style, I drag the same folder of images to each drive as I've completed metadata, editing and backup to DVD of all the files).
If $$ isn't an object, start thinking about using SSD devices for your system. They are pricy, but have no moving parts so dropping them and other issues of the mechanics aren't an issue, but there's always some other problem that we'll have to worry about.
Bottom line is that the drive is just one of the many factors involved with overall system speed and as always, YRWV.
