Why is USB so slow?

Question

I have two external drives connected to my iMac, one is FW and the other USB. I am using both to backup via SuperDuper! It takes about two hours via FW and three times that via USB (2.0). I am backing up about 220 gigs of data. Is there any reason the USB is so much slower, or is it something in my connections? Any help is appreciated. Thanks.
iMac 24-inch; MacBook Pro, various iPods including video   Mac OS X (10.4.8)  

Answer 1

From the Wikipedia articles on the two specs;
Though Hi-Speed devices are commonly referred to as "USB 2.0" and advertised as "up to 480 Mb/s", not all USB 2.0 devices are Hi-Speed. Hi-speed devices typically only operate at half of the full theoretical (60 MBytes/s) data throughput rate. The maximum rate currently (2006) attained with real devices is about half, 30 MB/s.[4] Most hi-speed USB devices typically operate at much slower speeds, often about 3MB/s overall, sometimes up to 10-20 MB/s.
FireWire 400 can transfer data between devices at 100, 200, or 400 Mbit/s data rates (the actual transfer rates are 98.304, 196.608, and 393.216 Mbit/s, ie 12.288, 24.576 and 49.152MBytes per second respectively). These different transfer modes are commonly referred to as S100, S200, and S400.